Museum of London archaeologists working on the £14.8bn Crossrail project have discovered 20 skulls in the historic channel of the River Walbrook.
The skulls, believed to be Roman, were found below the Bedlam burial ground established in the 16th century next to where workers are building a utility tunnel at Liverpool Street station.
Three thousand skeletons will be removed from the site next year during major archaeological excavations.
Roman skulls have been found along the historic Thames tributary and the River Walbrook throughout history and it was originally thought that they may have belonged to the rebels who were killed during the Boudicca rebellion against the Romans in AD 43.
But it is now thought that they have been coming from an eroded Roman cemetery in the Liverpool Street area. The latest skulls were found in a cluster indicating they may have been caught in a bend on the river.
Workers have also found wooden medieval structures at the site believed to have been part of the old walls of the Bedlam burial ground.
The Museum of London will be analysing the skulls over the next few months to find out more about the age, sex and diet of the people they belonged too.