The two companies will go to the High Court on April 25 over allegations that Watson Wyatt was negligent in valuing Crédit Lyonnais's UK pension scheme.

The bank has accused Watson Wyatt of overstating the scheme's assets and thus allowing Crédit Lyonnais to grant employees more generous benefits than it would otherwise have done.

Watson Wyatt is one of three actuarial companies that dominate the consultancy market, along with Hewitt and Mercer. As well as advising on the shape of company pension funds, the firms also provide investment advice to trustees.

A Watson Wyatt spokesman said: "I can confirm that a claim has been brought.

"We are confident that the work we undertook was entirely appropriate and we are vigorously contesting the ongoing claim," he added.

The consultancy refused to reveal the value of the claim but it is believed to be more than £1 million.

The spokesman said that Watson Wyatt would attempt to prove that the losses incurred by the bank were a consequence of management decisions that were taken without seeking the advice of the consultancy.

"Watson Wyatt is confident, based on the independent advice that it has received, that the court will decide that Watson Wyatt's work was appropriate and also that the court will reject the misconceived nature of the claim generally," the spokesman said.

This is the second time the consultancy has been implicated in such a case. In late 2003 Syngen, a California-based animal breeding company, took legal counsel because of advice that Watson Wyatt gave to the pension fund trustees at Dalgety, the pet food company that was broken up in the late 1990s. Syngen was created from the remains of Dalgety. The case did not go ahead.

Crédit Lyonnais was not available for comment last night.