Architect Frank Gehry (r) discusses his work with Prince Charles

The Sunday Times architecture critic Hugh Pearman praised the "soft reflective glitter" of the building.

He said: "If there is a safe way of removing the stains without damaging the super thin titanium sheets, then it should be done."

It is not clear what is causing the staining, since titanium is known to be a very resilient material.

The website for Timet Titanium, the US company that supplied the titanium for the building, says: "It is absolutely immune to environmental attack, regardless of pollutants."

But the staining might be nothing to do with the titanium itself.

Ms Abasalo said: "The staining may be the effect of pollution on layers of dust gathering on the titanium panels."

Meanwhile The New York Times reports Mr Gehry as saying that the problem arose when the building contractor allowed a silicon-based fireproofing sealant to spill onto the titanium.

Whatever the cause, cleaning the building will be no easy job.

It is so mathematically complex that the stone, glass and titanium curves were designed with the aid of computers.

A team of mountaineers is employed just to keep the glass curtain walls clean.

Getting to the staining with the equipment necessary to clear it will be a complicated process.

But Ms Abasolo added: "It is sure that it can be cleaned up and a Basque company will begin the work in the spring."