UPGRADES to a tube line said to be worth £4.4bn look set to descend into crisis over a funding row.

The Jubilee Line runs from Stanmore, through Wembley and on to central London but passengers have faced years of disruption due to engineering works that overran their original deadline.

Transport for London (TfL) and private company Tube Lines, responsible for the upgrades, are at loggerheads over a contract for seven-and-a-half years of work starting in July 2010.

Chris Bolt, as arbiter of public private partnerships involving the London Underground, today put the cost at £4.4bn, £400,000 more than TfL's original estimate but more than a £1bn less than the sum Tube Lines asked for.

London Mayor Boris Johnson said he did not want the taxpayer to pay the difference, sparking fears for the future of the project.

But Dean Finch, the company's chief executive, today said the arbiter's value was “not conducive to private sector involvement in the underground”.

Mr Bolt also said the Jubilee Line work could have been completed on time and on budget.

Councillor Daniel Brown, responsible for transport at Brent Council, said: “It's very frustrating for local people to hear that Tube Lines and London Underground could have completed the upgrade of the Jubilee Line on time and to budget and that Tube Lines' requests for more closures were unnecessary.

“I think mayor Boris Johnson should tell us why he allowed this to happen. Hopefully now Tube Lines and Transport for London will get on with the job and end the misery on the Jubilee Line due to works that should have already been completed.”

Mr Johnson said: “Londoners need and deserve a more frequent and reliable tube, delivered at an acceptable price.

“I welcome the arbiter's rejection of Tube Lines view of costs, and his recognition that they should be much closer to London Underground's.

“However, I am determined that any additional costs must not fall on London's fare payers and taxpayers.”

Mr Finch said: “The arbiter has acknowledged even at this level that London Underground has a stark choice to make – either to do less work or raise additional finance.

“However, this document is a draft and we have a further six weeks to make representations which we will do robustly.”