Stephen Byers has being accused of "jumping the gun" on the part-privatisation of the London Underground.

It is alleged he made an announcement to the Commons that the public private partnership (PPP) scheme would go ahead before key contract terms with bidders had been finalised.

The accusation is likely to trigger a last-ditch legal challenge from London's mayor Ken Livingstone, who remains bitterly opposed to the Mr Byers's plans for the Tube.

The transport secretary is accused of pushing ahead with the announcement despite a clear warning from the chairman of London Transport, Sir Malcolm Bates, that it would harm the public sector's bargaining position.

Sir Malcolm is said to have warned an early announcement would strengthen the hand of the companies still involved in negotiations and reduce value for money.

Court action

The warning was apparently contained in a letter by Sir Malcolm to Mr Byers on the day before his Commons announcement on 7 February.

A transcript of Sir Malcolm's letter reads: "I have told your (Mr Byers) officials we think this will damage the public sector's interests during the remainder of the competition."

Mr Livingstone delayed his threatened high court action against Mr Byers's plans by 24 hours on Wednesday.

But the release of Sir Malcolm's letter is likely to give his legal challenge fresh impetus.

Mr Livingstone told the BBC: "What he (Sir Richard) is basically saying is that if you go ahead and announce you have negotiated contracts and you go through the charade of consulting the mayor, it will strengthen the hand of the companies taking over the underground and we will get a worse deal."

Mr Livingstone and his transport commissioner Bob Kiley have waged a long campaign against Tube part-privatisation, which they say is expensive and potentially unsafe.

Responsibilities

Mr Byers was grilled about Sir Malcolm's letter on Wednesday as he gave evidence to the Commons transport committee.

Asked by Conservative MP Chris Grayling how it could be right for someone in his position to overrule the express wishes of London Underground, Mr Byers replied: "I have responsibility to the House."

Mr Byers also said it was true the contracts had not been finalised, but said that in his statement, he had said if there was a change which meant that contracts were no longer value for money, they would not proceed.

Under Mr Byers's proposals the Tube will be split into four main sections.

Two private consortiums, Metronet and Tube Lines Group, will be responsible for the maintenance and upgrading of the trains, lines and infrastructure under 30-year contracts.

The running of trains and operation of signals will remain under public control.