Oh dear what can the matter be, the American Ambassador's stuck inside Sotheby's

For trapped inside Sotheby's in the darkness was America's Ambassador to the UK with his two bodyguards - convinced they were about to come under attack.

The envoy, Robert Tuttle, was viewing an exhibition of Impressionist and modern art - including a work by Picasso that later sold for £7.4million - when the drama unfolded.

Fearing something was wrong when the lights went off, Sotheby's deputy chief in Europe, Melanie Clore, who was with the three men, escorted them to the front door - only to find it had been locked.

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The protection officers are said to have braced themselves for a terrorist attack and manoeuvred around their boss, while Ms Clore frantically guided them through the gloom to the back entrance.

The embarrassing incident happened last Saturday as the auction house in New Bond Street, London, was being closed at 5pm.

Ms Clore, who was escorting Mr Tuttle around, apologised for the blunder. And senior managers at Sotheby's were summoned to a hastily organised Sunday morning meeting to discuss what had gone wrong.

The Ambassador, 64, who with his wife Maria has been in London since 2005, is an avid collector of modern art and has decorated the US Embassy in Grosvenor Square with his own David Hockney paintings and sculptures by Alexander Calder.

The US Embassy refused to comment but Sotheby's spokesman Matthew Whiteman admitted that the front door had been mistakenly locked, though he would not say whether there had been a review of security arrangements.

He added: "The exhibition was closed, so our staff may have started to close some of the galleries around them.

"he protection officers were probably not aware this was happening."

The exhibition of works seen by the Ambassador provided Sotheby's with its most successful day of sales ever. The evening sale last Tuesday raised £116million, about £4million more than expected.