How the PPP contracts are split

Jubilee, Northern and Piccadilly Lines Bakerloo, Central and Victoria Lines District and Circle, and Metropolitan Lines

A spokesman for Mr Kiley told London's Evening Standard newspaper on Tuesday: "It is not clear if this offers anything new.

"The government had already offered step-in powers and they did not go anywhere towards meeting our safety concerns.

"We will be happy to look at these proposals when we are given the opportunity."

Under the PPP plan, three private consortia will be put in charge of maintaining the Tube infrastructure, with each responsible for a different section of the Tube network.

'Political noise'

London Underground boss Derek Smith complained that investment plans had suffered months of needless delay "due to political noise and expensive legal wrangling".

"It's now time to turn these proposals into a firm deal without further delay," said Mr Smith, who expected formal talks with bidders to resume in the middle of this month.

The "eleventh hour" contract changes stoked criticism, however, from shadow transport minister Bernard Jenkin.

He said: "If Mr Byers is still amending the PPP, he is admitting it is wrong... The right thing to do is to drop the discredited PPP"

Liberal Democrat transport spokesman Tom Brake sounded a similar note, saying: "The government has spent over three years and upwards of �100m on negotiating these deals.

"Only now do they acknowledge that safety was not guarded under PPP."

The changes failed to stem the campaign of the Rail, Maritime and transport union's Bob Crow.

"Political tinkering and papering over the cracks will not hide the fact that PPP will create a structurally less safe Tube that places profit before passengers," he said.