Two masked gunmen stormed one of London's most fashionable hotels and tied up a member of staff before fleeing with cash and mobile phones.
The raiders, speaking in Eastern European accents, burst into Dean Street Townhouse in Soho just after 3am, where they threatened an employee with a long-barrelled gun and demanded money, police said.
No shots were fired and no one was injured but one man was treated for shock.
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The hotel in Dean Street is popular with celebrities, and Kate Moss, Anne Hathaway and Brian Ferry are all said to have stayed there, while Gwyneth Paltrow cut the ribbon at its opening.
Met Detective Superintendent Jess Ruddell said hotel staff had been left "very shaken" by the raid and urged witnesses to contact police.
She said: "This is a very serious incident and we are working around the clock to identify those responsible.
"It was a very frightening experience for staff, who were threatened with a long barrelled weapon and were very shaken.
"The suspects are two men who were described as speaking with Eastern European accents and were wearing white hoodies.
"I want to appeal to any witnesses or anyone with information to contact us."
Dean Street Townhouse is part of the Soho House Group chain, which boasts a string of boutique hotels and private members clubs all over the world, including in New York, Hollywood and Berlin.
The four-storey Georgian townhouse has 39 bedrooms which are all individually designed with hand-painted Georgian wallpaper, according to the hotel's website.
It also has an impressive art collection which includes works by Turner Prize winner Tracey Emin, Mat Collishaw and Peter Blake, and is hung across the lounge and restaurant areas.
Staff at the hotel declined to comment on the raid.
Anyone who witnessed the incident or has any information is asked to call Metropolitan Police officers in Westminster on 101.
A Soho House spokesman said: " We can confirm that an incident took place in the lobby area of the Dean Street Townhouse in the early hours of this morning.
"Two male members of staff were on duty and no one was hurt in the incident. The matter is currently being investigated by the Metropolitan Police."