Metronet is responsible for improving two-thirds of the Tube

Metronet is investing �17bn over the next 30 years under a public-private partnership (PPP) scheme.

But it estimates there will be a �1.2bn overspend after seven-and-a-half years.

The private consortium claims LU should foot the bill, but LU said the overspend was due to Metronet's inefficiencies and so it should pay.

Economic review

But the firm says some of the blame lies with LU for forcing it to do work it was not originally contracted to do under the PPP contract.

Metronet chairman Graham Pimlott said failure to reach an acceptable solution had left them with no option.

"Where we have made mistakes our shareholders have borne the costs.

"However the PPP terms are clear - where additional spending is required to meet London Underground's demands then we are entitled to be paid," he said.

In February, London Mayor Ken Livingstone called for an independent review into Metronet's economic performance.

Additional payments

It followed a report by the PPP arbiter which found the company, which is responsible for the maintenance and renewal of two-thirds of the underground network, would overspend by �750m.

An LU spokesman said: "Metronet has not provided evidence to date that any additional payments would have been required by a competent organisation.

"It therefore remains of the belief that Metronet's projected cost overrun should be largely or entirely borne by it and its shareholders."