Some 20 fire engines and around 120 firefighters were called to the blaze in William Street on Wednesday afternoon (June 6).
London Fire Brigade (LFB) confirmed crews were tackling “a large fire at a hotel of 12 floors.
The fire is understood to be at the five-star Mandarin Oriental Hyde Park in Knightsbridge.
LFB confirmed crews were called to the scene shortly before 4pm.
“Fifteen fire engines and 97 firefighters and officers have been called to a visible fire in William Street in Knightsbridge,” said LFB in a statement.
“The brigade’s 999 control officers have taken more than 35 calls to the fire, which is producing a lot of smoke.
“The brigade was called at 3.55pm. Fire crews from Chelsea, Kensington, Hammersmith, Battersea and other surrounding fire stations are at the scene.
“The cause of the fire is not known at this stage.”
The hotel has just completed the most extensive restoration in its 115-year history and has 181 guestrooms, suites and public areas.
The work was designed to position the historic hotel as one of the best in the world.
The £695-room-a-night hotel includes 40 suites on its 12 floors including one within the turrets of the hotel, as well as the Dinner by Heston Blumenthal restaurant.