The building would have been one of the world's largest early churches. Only those of Italy and Trier in Germany were bigger.
Months of analysis of data from excavations on the site have revealed the existence of a 100m-long, 50m-wide building almost identical in design though slightly larger than the church of St Thecla in Milan, the largest church in the then capital of the Roman Empire.
It is even possible that this early cathedral was London's first St Paul's.
Archaeological research by David Sankey, a Museum of London archaeologist, indicates the Tower Hill building was constructed between AD350 and AD400.
Its most likely founder was a fanatically ambitious head of the Roman army in Britain, a deeply religious zealot named Magnus Maximus, which translates literally as "Great the Greatest".
He desperately wanted to be Emperor and succeeded in becaming ruler of most of western Europe from AD383 to 388, using Britain as his power base.
The Tower Hill cathedral was probably built in the late 370s or early 380s as part of Maximus's quest for Imperial glory. Its construction would have endeared the partly Christianised population to their benefactor.
The giant edifice was built of secondhand masonry (reused from other nearby earlier structures) and decorated in part with a wafer-thin veneer of black marble. Some architectural details were also pointed up in white marble, the walls were painted in red, white, grey, pink and yellow designs, and the floor was made of broken tile embedded in a sort of cement
The date of building, the probable political motive for its construction and a series of little-known ancient texts all suggest it may have been dedicated to St Paul, like London's later cathedrals.
In the second half of the 4th century, St Paul temporarily superseded St Peter in religious importance. This is expressed in mosaics and other art works throughout the empire where Paul replaces Peter at the right hand of Christ.
In politico-religious terms nothing could be calculated to enhance London's status more than to claim that the British capital was a Pauline Apostolic centre like Antioch, Ephesus, Athens and Salonica.
Although it is extremely doubtful that St Paul ever set foot in Britain, a legend to that effect seems to have been developed around the 4th century. The legend appears to have had its genesis in about AD100 when "Bishop" Clement of Rome wrote that Paul had evangelised "at the extremity of the West". But by the early 5th to 7th centuries, some churchmen were claiming that Paul had preached in Spain and "the islands of the Sea" and had "brought the laws of the gospel" to Britain and other lands.
Such legends would have been exploited to the full, if not invented, by ambitious politico-religious fanatics like Magnus Maximus, and apostolic aspirations may have played a key role in the choice of St Paul as London's patron saint.
In the end, Maximus was defeated in battle by the Eastern Roman emperor Theodosios. He was forced to retreat to his headquarters, the Italian city of Aquila, whose inhabitants arrested him and handed him over to the Imperial government. Maximus was executed, and his severed head was sent on a macabre tour of the empire.
London's first great cathedral was ultimately burnt down, probably by Anglo-Saxon barbarians in the 5th century. Its masonry was probably used to build parts of the medieval city wall and the Tower of London.Reuse content