The investigation centres on the alleged non-fulfilment of the terms and conditions of a contract

Metropolitan Police

A Scotland Yard spokesman said: "We can confirm that officers from the Metropolitan Police fraud squad are investigating allegations of fraud involving a London museum and a private contractor.

"The investigation centres on the alleged non-fulfilment of the terms and conditions of a contract."

An independent report carried out into the debacle by PriceWaterhouseCoopers criticised the museum for not acting quickly enough when the fraud became apparent.

Although the museum's trustees knew about the bogus stone in March 1999, their board did not debate the issue until the end of July 1999.

The Queen opened the new development in December 2000

The report also criticised the architect Foster and Partners and the stonemasons Easton Masonry for not detecting the problem.

The museum has refused to pay Easton Masonry the remaining �250,000 owed for their work on the South Portico.

Camden Council is currently considering whether to prosecute the museum over the Great Court affair because of alleged breaches of planning regulations.

The museum claimed the French limestone was as good as the English limestone, but art critics have disagreed.

Sarah Kent, Time Out magazine art critic, said: "The contrast between the mellow Portland stone and its replacement is glaringly obvious.

"It's a bit like spotting that the classical columns on the set of some Hollywood epic are made of polystyrene."