The move was announced by Transport Secretary Alistair Darling, who said he is not prepared to see the Public Private Partnership (PPP) for the Underground delayed any further.

He feared firms would refuse to press ahead with the contract because they would lose cash if the scheme was blocked by a legal challenge.

The project is being threatened by a proposed appeal to the European Court by Ken Livingstone who opposes the PPP.

The privatisation of the national rail network has been a disaster, and here is the government going the same thing again

Bob Crow, general secretary RMT

London Underground (LU) chief executive Paul Godier welcomed the move.

"For my customers this is really good news. It means we're in the home straight now, of really transforming and modernising the Tube to be fit for a world city," he told BBC London.

Two actions in English courts have already failed to prevent the PPP deal.

Mr Darling told the Commons in a written answer that he would not transfer London Underground to Transport for London (TfL) while any legal challenge was outstanding.

'Legal squabbles'

He said he expected the Tube Lines contract to maintain the Jubilee, Northern and Piccadilly lines to be operating in January.

The contract for the rest of the system, to be maintained by Metronet, would come into effect in the spring.

Ken Livingstone is opposed to Tube PPP

Mr Darling told BBC London: "The London Underground desperately needs money going into it.

"I would have thought the Mayor would have done far better working to make sure the money goes into the Tube rather than indulging in further legal squabbles which, after all, people of London have to pay for."

The PPP deal promises improvements costing �17bn over 30 years.

Private companies carry out maintenance and upgrades and in return will receive a monthly payment from LU for the work.

'Same mistakes'

Mr Livingstone claimed the government had reneged on its promises.

"The government promised that this scheme would deliver a transfer of risk to the private sector and instead it has simply become a transfer of risk to Londoners."

The biggest Tube union, the Rail Maritime and Transport (RMT) union claims PPP will make the same mistakes as on the rail network.

RMT general secretary Bob Crow said: "The privatisation of the national rail network has been a disaster, and here is the government going the same thing again to the Tube - whatever the cost."