Atkins owns a fifth of Metronet and, if the overspend is apportioned pro rata, the shareholders will have to stump up £150 million. The other shareholders are Balfour Beatty, Bombardier, EDF Energy and RWE Thames Water.

Even that sum may be dwarfed by the collateral damage to Atkins’s reputation caused by its association with a company that generates negative headlines almost daily. London Underground this week announced that Metronet was facing a £1 million penalty for the failure of a signalling system that forced the Central Line to close in the morning peak on Monday.

On the same day, Metronet’s late-running engineering works caused chaos on the Circle and District lines, triggering another £100,000 in penalties.

Atkins can hardly distance itself from Metronet’s woes because it is intimately involved in the running of the company.

It recently sent another 150 of its staff to work at Metronet, bringing the total number on secondment there to 750. This in itself represents a significant opportunity cost because they could be working on more profitable metro schemes overseas.

Atkins is also a 25 per cent shareholder in Trans4M, which has contracts with Metronet to deliver its station upgrades. Delays to these upgrades are Metronet’s biggest headache. It promised to complete 35 stations by the end of March but delivered only 14, incurring major penalties.

Chris Bolt, the Tube arbiter, last week condemned Trans4M’s poor performance and Metronet’s inability to manage the contracts. He was particularly critical of Metronet’s supply chain, under which the company’s shareholders have the contracts for nearly all the work. It is hard for Metronet to hold its contractors to account when they are also its shareholders.

By contrast, Tube Lines, the other Tube maintenance company, held open competitions for its contracts and many were awarded to outside companies.

Mr Bolt also indirectly criticised the decision to appoint Keith Clarke, chief executive of Atkins, as chairman of Metronet. He called on Metronet to improve its governance by “revisiting the structure of the board to provide greater independence, for example including a chairman who is independent of the tied supply chain”.

Mr Clarke has agreed to step down but has declined to set a date. In the longer term, Metronet may still be a cash cow for its shareholders, but only if it can persuade Mr Bolt that it is economic and efficient. He has stated that it is not economic and efficient now but Metronet is playing a longer game. As long as it wins Mr Bolt’s approval by 2010, it can demand higher payments at the 7½-year break point in its 30-year contract.

Atkins may yet find itself protected by the loopholes in the contract that protect Metronet when costs rise for reasons beyond its control.

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