Transport Secretary Lord Adonis delivered a serious slapdown to Mayor Boris Johnson today in a funding row and threatened to take over Transport for London.

Johnson had demanded that central government step in with the £400m needed to bridge a budget gap in maintenance and upgrades to the Jubilee, Northern and Piccadilly lines - because the government imposed Private Public Partnership on London.

That gap is the product of an independent arbiter, contractor Tube Lines said TfL should pay it £5.75bn, TfL would pay £4bn; the arbiter put the cost at £4.4bn.

The Labour Lord fired off a furious letter, the contents which have been revealed today by the Department of Transport.

Lord Adonis goaded Johnson:

"If Boris wants me to take charge of TfL then he should say, and I would start with more sensible priorities like not cancelling the Western congestion charge zone and not replacing a modern bus fleet needlessly- both of which are costing Londoners hundreds of millions of pounds which could be spent on upgrading the Tube."

He questioned the Mayors ability to balance the books and reminded Johnson that government funding to TfL had more than doubled in the last decade and would reach almost £3.3 billion a year from April.

He said:

"Under devolution, it is for the Mayor and TfL to deliver the Tube upgrades within their generous budget - not for me to bail them out if they fail to do so.

It is not acceptable for you to ask Government to bail you out if you cannot balance your accounts."

He also pointed out that the Underground's last two major projects, the Central Line and Jubilee extension went 30% over budget.

A final decision on the upgrade bill will be taken on March 4th by an independent expert.

The issue is becoming intensely political as the Mayor's office plot to bounce out the last remaining edifice of PPP from London Underground and at the same time deliver a damming blow to Gordon Brown's credibility -  as Chancellor he fought former Mayor Ken Livingston through the courts, at public expense, to force the 30-year contract on the capital.

Metronet, the primary contractor collapsed with debts of £2bn in 2007.

Tube Lines receives a guaranteed 26 per cent return on its investment.