London mayor Ken Livingstone has issued a blistering attack on the government's controversial public-private partnership plans for the Tube.

Following transport secretary Stephen Byers's announcement on Thursday, Livingstone questioned the independence of the Ernst and Young report into the PPP contracts and warned he is likely to take his fight to the courts.

He claimed the figures used to assess the bids had been manipulated in favour of the private bids and accused Byers of stifling the debate over the issue.

"The authors of the so-called `independent' report on which Stephen Byers has based his claim that the PPP represents value are the consultancy firm Ernst and Young. I have today been informed that this company is anything but `independent' on this issue," Livingstone said.

"Ernst and Young are in fact the auditors for key companies in two of the successful PPP consortia - Bombardier and Jarvis. This appears to be a clear conflict of interest and on these grounds alone this report should be disregarded."

The mayor claimed the contracts repeated the key failings that had dogged Railtrack.

'The PPP will saddle the travelling public and council tax payers of London with huge and unquantified liabilities while replicating the key mistakes of rail privatisation on the Underground," he said.

He gave a clear indication that he is likely to return to the courts in a bid to get the decision overturned.

"As mayor, I have a clear duty to uphold London's interests by taking all necessary steps to oppose this scheme," he said.

"If my lawyers advise that these should include further legal action I will have no hesitation in taking it to protect London's travelling public from what in years to come will be understood by everyone to be a folly as great as the debacle we have seen on the national rail network."

His comments were echoed by several Labour backbenchers who lined up in the Commons debate on Thursday night to voice their anger.

Gwyneth Dunwoody described the plans as "manifestly unworkable" and indefensibly incompetent.

London Labour MPs also added their criticism. Diane Abbott accused the government of "breathtaking arrogance" and Neil Gerrard claimed going ahead with the deal would cost the party votes.

Andrew Bennett, the Labour MP for Denton and Reddish, described the deals as a "pig in a poke" and Harry Cohen, Labour MP for Leyton and Wanstead, demanded to know who was accountable for the scheme - the mayor of the minister.

Tory transport spokesman claimed Byers statement had been allotted the 7.00pm slot usually reserved for war or catastrophes. He said the announcement was the latter.

The formal decision to proceed was made by London Underground board, which on Thursday informed transport secretary Stephen Byers of its decision to approve the deals with the private sector consortiums bidding to take over the maintenance of parts of the Tube network.

London Transport Board members voted unanimously in favour of a recommendation from London Underground to proceed with arrangements for all three competitions with the two consortia, Metronet and Tube Lines Group, which will take over responsibility for repairing the Underground network.

In a Commons written answer, Byers said that the final decision would be conditional on the Health and Safety executive approving the scheme and on no substantial changes being made to the costs of the contracts.

Byers said the PPP would offer value for money. "I have taken separate independent advice from Ernst and Young. Ernst and Young have confirmed to me that the process followed by London Underground's evaluation team has been suitably robust and that London Underground's recommendation that the PPP proposals deliver value for money is a subjective one which is supported by its analysis," he said.

London Transport chairman, Sir Malcolm Bates, said that the decision demonstrated the board's belief that the deal was both safe and offered value for money.

"The PPP has been subjected to intense scrutiny throughout its development. It would not have reached this point unless the LUL and LT Boards had been confident at each incremental stage that they would be able to answer this last question affirmatively," said an LU report.

The deal will not be signed until the 20 day period for consultation has taken place with Transport for London, the mayor's transport body headed by Bob Kiley, which has vigorously opposed the plans.

London's Labour backbenchers had forced the transport secretary to bring the decision forward so that the issue does not dominate May's local elections.

But Thursday's announcement is unlikely to be the end of the saga.

Susan Kramer, the Liberal Democrat board member on Transport for London, said the decision could "consign Londoners to 30 years of Tube hell".

"Byers is proposing to sign a PPP that commits private companies to provide minimal improvements, completely inadequate to cope with London's existing, never mind its growing population," said Kramer.

"As the ultimate insult, Londoners and the British taxpayers are being asked to pay through the nose for the privilege of a hellish ride for the next 30 years."

In the Commons on Wednesday, Tony Blair said the PPP was "the right thing to do".But his remarks went against the recommendation by the Labour-dominated transport select committee which said that the government should scrap its plans.

Cost effectiveness of the proposed public private partnership could never be accurately evaluated, it concluded in a report which caused embarrassment to ministers.

The MPs concluded that expensive disputes over contracts and between employers and staff were "inevitable".

The report called for the government work closely with Livingstone and Transport for London, who would eventually have responsibility for investment decisions and safety issues.

The report also called for the government to subsidise the service to meet the capacity increase target of 15 per cent over the next 20 years.

The MPs said the subsidy would be justified by "the potentially vast cost to London and the nation's economy of failing to meet that target".