The Cathedral of St Thomas of Canterbury is a light, spacious church with a complicated plan, built in four principal phases.

The oldest part of the church, the original aisled chancel (now the Chapel of St Thomas), dates from c.1180-1190. The plain exterior is of rubble with ashlar dressings, and the roofs are covered in red tiles. A blocked oculus in the E gable once lit the roof space. Internally, several of the aisle capitals are late Romanesque or Transitional in character, but the rest of the ensemble – which includes stiff-leaf – is Early English. Brief entries on the Early English carvings are included here to contextualise the Romanesque or Transitional sculpture

Each chancel arcade is composed of four bays of pointed arches, grouped into pairs beneath broad round-headed arches. These correspond to the four vaulted bays of the aisles and the two vaulted bays (rebuilt in wood and plaster c.1843) over the main vessel. Above the arcades, a clerestorey incorporates a passage which continues across the E wall, through a triple-light window. To N and S, each clerestorey bay comprises a screen of three pointed arches and a single lancet. The en-delit shafts (with annulets [shaft rings] to the E and W responds) carry round capitals.

The transepts of c.1190-1230 (now the Lady Chapel to N and the Holy Martyrs Chapel to S) largely survive, although the roofs and much of the W walls were rebuilt in the 17thc (see History, below). Plaster has been removed from the S wall of the S transept, exposing evidence for medieval vaulting, but both transepts now have flat ceilings (renewed 1992).

In the S transept, a low doorway at the S end of the W wall serves a vice (stair) to the clerestorey passage, which continues through simple lancet windows to E and S. Behind a blocked pointed arch in the E wall lies the vestry (now flower room) of 1828, which is accessed from the S chancel aisle.

The clerestorey passage does not continue into the N transept, which is slightly longer than the S transept. It is lit by irregularly positioned lancets to N and E. The upper lancets, including a triplet in the E wall, have cusped heads. A pointed arch in the E wall contains a double lancet with a quatrefoil in the spandrel (almost a form of plate tracery). Between this and the chancel aisle, a mid-13thc wall painting (uncovered c.1904) occupies a trefoil-headed niche. To the W, a wide pointed arch (built 1935-38) opens into the outer quire aisle beneath a blocked lancet.

A small cloister and associated single-storey buildings (bishop’s room, vestries and sacristy) butt against the church to the NW of the N transept. This complex, dating from 1935-37, was built of coursed rubble with a flat roof, except for the vestry range, which is covered by a hipped tile roof.

W of the transept is the aisled quire (choir; former nave) and the central tower (former W tower) of 1683-94. The tall, simple columns of the nave arcades have Tuscan capitals and carry round arches, topped by a modillion cornice. Since there is no clerestorey, the central space is lit by dormer windows which puncture the barrel vault. The aisles, which were engulfed by outer quire aisles in 1935-38, are also lit through their roofs. The massive tower carries an octagonal cupola topped by a lantern and a weathervane. Internally, arches punched through the E and W sides create a view of the quire from the nave. The transepts to N and S of the tower ('the tower transepts') were added in 1935-38.

A four-bay aisled nave (1938-39; 1990-91), surrounded by a flat-roofed ambulatory, comprises the W end of the cathedral. It is built of coursed rubble with ashlar dressings. An 18thc doorway has been repositioned between bays 3 and 4 of the N ambulatory. The W bay of each aisle (1990-91) rises the height of the nave, under a hipped roof. The main vessel is lit by four-light clerestorey windows with segmental arches. The round-headed entrance and rose window in the W façade are positioned between two square towers with ogee roofs. Despite being built in 1990-91, this façade has the character of an early Romanesque westwork.

According to the Domesday Survey of 1086, three manors existed on Portsea Island (Buckland, Copnor and Fratton) but no church was recorded at any of them. It is often claimed that the town of Portsmouth was founded by the Norman merchant Jean de Gisors, who bought Buckland from the de Port family. In 1180, soon after this purchase, de Gisors granted land (‘Sudweda’) at the southern end of the manor to the Augustinian canons of Southwick Priory – who already controlled the parish church of St Mary, Portsea Island – so that they could build a chapel of ease dedicated to St Thomas of Canterbury (Thomas Becket), who had been martyred in 1170 and canonized in 1173. The chancel was consecrated in 1188, and the churchyard (with grant of burial rights) and two transept altars in 1196. Progress may have stalled in 1194, when de Gisors forfeited his lands, including Portsmouth, to Richard I. The transepts and central tower were probably built in the period 1190-1230, followed by the nave.

The chapel became a parish church in its own right in 1320. Dendrochronology suggests that the chancel was reroofed in 1389-1421 (Bridge 2012). In 1543 the possessions of Southwick Priory, including St Thomas’s, were granted to Winchester College.

The earliest known view of the church, on a map of c.1585 in the Hatfield House Archive, may be relatively accurate (reproduced in Conservation Management Plan, 2024, 5). It shows a cruciform church with a two-bay aisled chancel, a transept, a central tower, a four-bay aisled nave and a S porch. The churchyard is surrounded by walls to E, N and W, and bounded by houses to S (High Street). The S transept is depicted with broad buttresses and three levels of fenestration: two plain lancets in the lower level, and a single, taller lancet, in each of the upper levels. This corresponds with what survives today, although the upper lancet is a small window serving the roof space. The tower is shown with an oblong bell-opening (divided into six openings) and a flat roof topped by a hoist.

The tower and nave were badly damaged by Parliamentary bombardment in 1642. Between c.1683 and 1694 the nave and aisles were rebuilt with three bays covered by a catslide roof which was later pierced by hipped dormers. Additionally, a massive W tower was erected, the remains of the medieval crossing tower were removed, and the transepts were repaired and reroofed. The chancel was also affected: a new chancel arch was built and a flat ceiling replaced stone vaulting (assuming it had survived) beneath the timber 14thc roof.

An octagonal cupola was added to the tower, which was used as a navigational aid, in 1702-03. It was augmented by a gilded ship weathervane in 1710. A W gallery was constructed in the nave in 1706, with stairs flanking the tower. An organ, installed in the W gallery in 1718, dominated the interior. The galleries were extended along the aisles and around the transepts in 1750. A porch was built on the N side of the chancel in 1809 (reconstructed 1895) and a vestry was erected as part of a repair programme in 1828.

In 1843 the church of St Mary, Portsea Island, was completely rebuilt to designs by the local architect Thomas Ellis Owen (1805-62). In the same year Owen restored the chancel of St Thomas’s. The flat ceiling was replaced by wood and plaster vaulting (in the style of the stone quadripartite vaulting surviving in the aisles), the 17thc chancel arch was replaced and the E transept galleries, which had masked the chancel aisles, were removed. The installation of various screens and galleries appears to have damaged the 12thc stonework. The Hampshire Telegraph noted that ‘the capitals of the shafts, which had either been chipped off or covered in plaster, have been restored’. It was also noted that ‘the stone windows of the chancel aisles have been restored in a correct form’. Accumulations of whitewash were removed from the Purbeck columns and the 15thc font was recut.

In 1902-04 Thomas Graham Jackson (1835-1924) undertook substantial work in the nave and chancel. Concerns had been raised about the number of burials beneath the church, and also about the stability of the foundations. Floors were lifted, and both nave and chancel were underpinned. At the same time, some of the stonework in the chancel, including several arches and three capitals, was replaced. Electricity was installed and a new boiler room was created under the vestry. The remaining transept galleries were removed.

Soon after the church became a pro-cathedral in 1927, the diocesan architect, Sir Charles Nicholson (1867-1949) made minor alterations and additions. In 1929 land was acquired to the W, securing the status of the site, which became a cathedral in 1935. Anticipating this, in 1932, Nicholson was commissioned to design a new nave capable of seating 1800 people. He started, in 1935-37, by rearranging the N side of the church around a new cloister. This engulfed the 18thc nave (now to become the quire) with an outer aisle and a transept on the N side of the W tower. These were balanced by similar additions to the S in 1937-38. Work then began on a five-bay aisled nave which was to have a W front with two towers. Two superimposed arches were punched through the E and W sides of the 18thc tower to create a view (somewhat restricted) of the quire. Three bays had been completed by the end of 1939, when work was stopped by the outbreak of war.

A brick wall was erected to enclose the incomplete nave and aisles, but rebuilding did not immediately resume after the war. The Cathedral considered discarding Nicholson's design. In 1966 a scheme by Seely & Paget, with Pier Luigi Nervi as consultant engineer, was published. It proposed a dramatically curved W end, clad in glass or stone. Pevsner & Lloyd, who dismissed Nicholson’s work as ‘inert traditionalism’, were enthusiastic, describing the design in detail and expecting its completion in 1969 (Pevsner & Lloyd 1967, 402-3). The project, however, never left the drawing board. It was only when the brick enclosure was deemed structurally unsound in the 1980s that the Cathedral proceeded with an alternative design, developed by Michael Drury. Built in 1990-91, this was a truncated version of Nicholson’s scheme. It involved one additional nave bay, a new façade, and two W towers.

Architecturally, the chancel of Portsmouth Cathedral has been compared by many authors with the E end of Chichester Cathedral (principally the retrochoir, post-dating the fire of 1187), the choir of Boxgrove Priory (c.1200-20) and the choir of New Shoreham (begun c.1170), all in West Sussex. Like Portsmouth, these sites have sculpture which is classed ‘Transitional’.

Unfortunately, no comparative material survives from St Thomas’s parent house, Southwick Priory, which was located just N of Portsmouth. More locally, the important parish church of St Mary’s, Portsea Island, has been completely rebuilt. Capitals with spear-shaped (or spade-shaped) leaves can be seen in several churches within the region, such as North Hayling and Havant.

In terms of phasing, Pevsner & Lloyd have suggested that the N side of the chancel was built first, noting that the mouldings to the S were ‘subtler and more undulating’ and that the capitals were plainer (Pevsner & Lloyd 1967, 396-397). However, the same foliage capital type (splayed leaves) recurs to the S, so the argument is not conclusive. The two aisles appear to be largely contemporaneous. The outer envelope of the structure may have been built before the arcades: it is worth noting that round, moulded capitals only feature in the peripheral walls at the E end of the N chancel aisle and E respond of the N chancel arcade. Capitals with upright spear-shaped foliage, representing the continuation of Romanesque traditions alongside the emergent new aesthetic, occur at the W end of the S aisle and on the SE aisle vaulting shaft.

In its entirety, the chancel is usually dated c.1180-90 (or, more narrowly, c.1185-88). The upper levels of the main space, with their mature crockets and windblown stiff-leaf, may be slightly later in date, perhaps even as late as c.1200. J. Randall has suggested that the two human heads may have been reused from a different context (information from R. Baxter).

The transepts are generally dated to c.1190-1230. Observing the presence of cusped lancets in the N transept, Pevsner & Lloyd suggested that the S transept was built first (Pevsner & Lloyd 1967, 394 and 398). The continuation of the clerestorey passage into the S transept strengthens this theory. The fact that the transepts have different fenestration and lengths may suggest that they were built piecemeal over an extended period.

Several capitals and impost blocks appear to have been restored with inserts or completely replaced c.1843. The three capitals replaced in 1902-04 were those on the black marble shafts to NE, NW and SE. Pevsner & Lloyd noted in 1967 that the SW capital (now pale ashlar) was of Purbeck marble (Pevsner & Lloyd 1967, 395), and it certainly appears to be a dark colour in mid-C20th. photographs (Historic England Archive, red boxes). It may have been replaced since 1967.

A number of loose architectural fragments in an Early English style were photographed in 1963 (HE Archive). Their current whereabouts is not known.