There have been no traces of anthrax or bubonic plague on human bone fragments found during tunnelling work for the £16bn Crossrail project.
The human remains had been found in May in a deep exploratory bore hole in Farringdon, central London.
The threat of diseases being released was raised by Lord James of Blackheath at the passing of the Crossrail Bill.
The human fragments had been sent to the Health Protection Agency for testing, which found no traces.
Deadly anthrax spores can lie dormant for centuries but if disturbed can spread through the air.
The bacteria which causes bubonic plague can also survive in plague pits.
The dig uncovered mostly animal and some human bone remains which had been sent to the Health Protection Agency for testing.
The work on the exploratory tunnelling had been halted until the negative results had been returned, a spokeswoman for Crossrail had said.
Lord James of Blackheath told the House of Lords Crossrail Bill select committee that 682 victims of anthrax had been brought into Smithfield in Farringdon with some contaminated meat in 1520 and had been buried in the area.
Crossrail said the risk from the ancient diseases was very low but excavation would be done correctly.
Keith Berryman from Crossrail said: "I think it is a legitimate concern.
"The thing about London is that it is well mapped so we know where the plague pits are and we can take special care.
"If we find any remains they are sent for testing."
Crossrail will link towns to the east and west of London with Heathrow, the West End, and Canary Wharf.
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