Tube bidder Metronet is negotiating a special clause in its contract in case further work is needed on the Central Line after it signs its PPP deal.
The consortium, which includes contractor Balfour Beatty and consultant Atkins, will not foot the bill for special works after the Chancery Lane crash last month.

A London Underground source said: "The work is being termed as a special project, which will be outside the PPP deal."

Metronet is still expecting to reach financial close on its two deals in early April, although doubt emerged over the transfer of responsibility for the Tube from the government to Transport for London.

London mayor Ken Livingstone claimed this week that the transfer may not happen until December. Building revealed last month that the handover, originally expected to be in the summer, could be delayed to later in the year.

Livingstone said: "This would be an absolute disaster. My concern is with a management on the way out, their eye will not be on the ball as far as safety is concerned."