Archaeologists working on the £14.8 billion Crossrail project have unearthed 20 skulls, believed to have once belonged to Romans.
The cross-London scheme has already led to a number of exciting discoveries with the latest coming while workers were building a utility tunnel at the Liverpool Street station site.
Working under the direction of Crossrail's archaeologists, the construction workers carefully removed the human skulls and Roman pottery, found in the sediment of the historic channel of the River Walbrook.
The skulls were found below the Bedlam burial ground established in the 16th century, where 3,000 skeletons will be carefully removed during major archaeological excavations next year.
For safety reasons, the archaeologists have had to leave the archaeology work to the tunnellers as the skulls were up to six metres below ground.
Roman skulls have been found along the historic Thames tributary the River Walbrook throughout London's history. This led to speculation they were heads decapitated by Queen Boudicca's rebels during the rebellion against Roman occupation in the 1st century AD.
However, later archaeology suggested that the River Walbrook possibly eroded a Roman cemetery under Eldon Street in the Liverpool Street area and the skulls and other bones had been washed downstream.
The latest skulls were located in clusters indicating they were caught in a bend in the river. The location of the skulls indicates they were washed out of the burial ground during Roman times.
Lead archaeologist Jay Carver said: "This is an unexpected and fascinating discovery that reveals another piece in the jigsaw of London's history.
"This isn't the first time that skulls have been found in the bed of the River Walbrook and many early historians suggested these people were killed during the Boudicca rebellion against the Romans."
He went on: "We now think the skulls are possibly from a known Roman burial ground about 50 metres up river from our Liverpool Street station work site. Their location in the Roman layer indicates they were possibly washed down river during the Roman period."
The tunnellers have also discovered wooden medieval structures believed to have been part of the walls of the Bedlam burial ground.
The Museum of London Archaeology will analyse the finds over the coming months and hope to find out more about the age, sex and diet of the people associated with the Roman skulls.