An enormous Roman sarcophagus has been discovered during excavations at St Martin-in-the-Fields in Trafalgar Square, revealing that Roman London extended well beyond previously known boundaries.

Weighing 1.5 tonnes and containing a human skeleton, the limestone coffin dates from the late 4th or 5th century. It was found a couple of kilometres from the site of the Roman town known as Londinium.

The date of the sarcophagus makes the find all the more exciting, as the buried man may have been a contemporary of the Church’s patron saint, St Martin, a Roman soldier who converted to Christianity and died in AD397.

Other finds include a Roman tile kiln, dating from AD400-450, indicating that a significant Roman building existed near the site, and Anglo-Saxon burial remains from the beginning of the 7th century.

They reveal for the first time that the church of St Martin-in-the-Fields has been a sacred site for far longer than anyone realised.

The Vicar of St Martin’s, the Rev Nicholas Holtam, said: “These discoveries take us right back to St Martin himself . . . This find is extraordinarily moving.” Perhaps, he joked, these are the bones of St Martin himself”.

The position of the sarcophagus suggests a Christian burial.

The sarcophagus was unearthed during excavations by archaeologists from the Museum of London, as part of St Martin’s £36 million renewal programme. It was found 10ft (3m) below pavement level, next to the main building.

Tests are yet to be conducted on the skeleton. It is likely to have been a high-status individual, though, as the limestone would have had to be transported from Oxfordshire or Northamptonshire.

The skeleton is missing its head, which is thought to have been removed from the grave when the lid of the sarcophagus was damaged by workmen building a sewer in the Victorian period. They may also have removed grave goods.

The man, who was 5ft 6in tall, died in his forties. He suffered three cracked ribs, which had healed. He also seems to have suffered from a bad back.

Gordon Malcolm, of the Museum of London Archaeology Service, said: “It will offer rich possibilities in telling new stories of London and Londoners during the decline of Roman influence before the arrival of the Angles and Saxons later in the 5th century AD. This elusive period is poorly understood, and this find will redraw the map of Roman London studies.”

Taryn Nixon, director of the Museum of London Archaeology Service, said: “This is an extraordinary eye-opener for us. We thought that we had an impression of what was going on in the area. All of a sudden we have had to rethink Roman London.”

No other tile kilns have been found in Central London, and the kiln is the latest-dated structure from Roman London to have been found thus far.

The site lies at the western edge of what was once the large middle-Saxon town of Lundenwic. Among the most attractive finds made at the site are grave goods from Anglo-Saxon burials, including an exquisite gold pendant.

The dig has encompassed the dilapidated 19th-century vaults that previously housed many of the activities at St Martin’s — including social care, rehearsal space and day centres for the Chinese community. New modern spaces will be provided for these activities.

Reflecting on the regeneration work that has given rise to the discoveries, Mr Holtam said: “How wonderful that in doing work to secure the future of St Martin’s we have unearthed its unknown past.”