A Roman sarcophagus discovered near Trafalgar Square could lead to the map of Roman London being redrawn.
The skeleton's head was probably removed by Victorian workmen
The limestone coffin containing a headless skeleton was found during excavations at St-Martin-in-the-Fields Church, central London.
The find, which dates from around 410AD, lies outside what were the city walls of Roman London.
Archaeologists previously thought Westminster possibly contained Roman roads but not sacred buildings.
Taryn Nixon, director of the Museum of London Archaeology Service, said: "It means that perhaps St-Martin-in-the-Fields has been a sacred site for far, far, far longer than we previously thought.
"This gives us an extraordinary glimpse of parts of London we haven't seen before, particularly Roman London and Saxon London.
"All of a sudden we're having to rethink what Roman London really was. This work has literally stopped us in our tracks and given us a new phrase, Roman Westminster."
Vicar Rev Nicholas Holtam said: "I can't tell you how thrilling it is to have discovered these finds.
"St Martin's history tells us that the earliest church that we know of on the site was there in 1222, but these discoveries take us way before that.
"It's certainly a sacred site, possibly a Christian site, going right back into the late Roman period."
It is thought the skeleton's head was removed by workmen building a sewer during the Victorian period.
Excavations began at St-Martin-in-the-Fields in January 2006 as part of £36m renovations at the church.