The mayor says cost overruns are unacceptable.

The announcement follows a report from Ken Livingstone's advisors that Metronet's performance remains poor.

A report by the Public Private Partnership (PPP) Arbiter in November found Metronet will overrun by �750m.

Metronet is responsible for the maintenance and renewal of two thirds of the London Underground network.

No improvement

"Following a further report to me on Metronet's performance from London Underground Managing Director Tim O'Toole I have reluctantly come to the conclusion that an Extraordinary Review by the PPP Arbiter is called for," said Mr Livingstone.

"He is the only person to decide how much of the cost overrun must be borne by the Metronet shareholders."

He added: "I will not stand for any failure to deliver this renewal, or allow the costs of Metronet's failure to perform to fall on London."

Mr O'Toole said: "We have consistently made clear our frustration with Metronet's overall performance in the delivery of projects and in track maintenance, but unfortunately I have yet to see any real evidence of improvement."

But in response, a spokesman for Metronet said: "The question of 'additional costs' are provided for in the PPP contracts."

"This is not a risk-free project and given the sheer scale of the Tube's renewal programme following decades of under-investment, the originators of the PPP provided a proper mechanism to deal with the question of additional and unforeseen costs."