The National Audit Office has told MPs to question "letters of comfort" issued by the transport secretary to private companies taking over responsibility for London's underground tube network.

Stephen Byers had failed to give parliament adequate time to consider the potential liabilities for taxpayers if the tube privatisation crashed, warned Whitehall's spending watchdog.

Byers's failure to give MPs 14 days advanced notice of the guarantees was leaked by Lib Dem David Rendel, who sits on the Commons public accounts committee.

Only four days notice was given despite the government having had the information since February 7 and, Rendel believes, the move is an admission that taxpayers will take the risk of bankruptcy for private companies.

"This blows a huge hole in the secretary of state's justification for the PPP. It is no surprise that Stephen Byers is trying to sneak this through without the normal 14 day consultation period," he said.

"Yet again Stephen Byers has shown himself unfit for high office."