Brad Pitt was promoting his new film Oceans 12

The small fire is thought to have been in the Mayfair hotel's kitchen run by celebrity chef Gordon Ramsay.

Four fire engines were called to the scene and all the guests left the building.

Mr Pitt and Mr Damon were due to give a press conference about the movie sequel Oceans 12.

A London Fire Brigade spokesman said crews were called at 1100 GMT and that the fire was under control but it was too early to say exactly where it started.

Sandra Burr, 58, said she was standing outside the hotel in the hope of getting an autograph from Jim Carrey, who is also in London promoting his film, Lemony Snicket's A Series of Unfortunate Events.

She said: "I saw Jim Carrey, Mick Jagger and Matt Damon all leave the building."

Guests were later allowed back inside the 200-room hotel.