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Bosses of the international hotel chain recently announced the end of renovation work "designed to confirm this historic hotel's position as one of the best in the world".
Plumes of smoke were seen rising from the five-star Mandarin Oriental in Knightsbridge just before 16:00 BST.
A total of 36 hotel guests and 250 members of hotel staff were evacuated.
The blaze reached several floors but was quickly extinguished, the London Fire Brigade (LFB) said.
At the height of the fire, about 120 firefighters and 20 engines were sent to the the 12-storey building in William Street, near Hyde Park.
The cause of the fire is not yet known and firefighters are likely to remain at the scene through the night, an LFB spokesperson said.
Both the ambulance service and the Met Police attended, although there were no reports of any injuries.
The Glasgow School of Art has been devastated by a huge fire, only four years after parts of the building were destroyed by a smaller blaze.
Turning back the clock 200 years with a river restoration costing nine million pounds could prevent ‘catastrophic’ flooding in Budleigh Salterton.
Mr/Mrs Jacobson says...