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

Sarah Kent, art critic

The report, commissioned by the museum from PriceWaterhouseCoopers, found the museum "was deceived" by the switch.

"It did not react fast enough to manage the situation effectively," it said.

"The museum, its management and trustees have suffered reputational damage as a result."

Culture Secretary Chris Smith insisted that museum trustees commission the report.

When the switch of the stone was uncovered, the museum consulted English Heritage and Camden Council before deciding to go ahead, so as to avoid delays and further costs.

The report said there was no attempt on the part of the museum to cover up the affair, but said that the management and trustees should have acted earlier to avoid adverse publicity.

But it added the problems relating to the South Portico "have been blown out of all proportion by an exaggerated campaign in a small section of the press".

The Queen opened the new development in December 2000

The museum has accepted the criticism, but argues that the problems do not detract from the project as a whole.

"We are pleased the Great Court Project has been completed successfully and opened to wide acclaim and that the British Museum is now receiving 40% more visitors," said chairman Graham Greene.

The museum had 320,000 visitors in the month after the opening of the Great Court on 7 December.

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."

But Isabelle Allen, editor of The Architect's Journal said: "I think it's stunning. I have problems with other parts of the building, but not the Portico and the roof."

Foster and Partners laid the blame with the contractor, pointing out that they are not equipped to deal with fraud routinely.

They said: "If a credible stone mason...has determined to embark upon a dishonest and deceitful course, it is going to be extremely difficult to prevent such a person exploiting procedures which are not fraud proof."