Flames could be seen in footage taken at MNKY HSE within the upmarket Mayfair space.
The London Hearth Brigade mentioned it believed an sparkler, believed to be in a cocktail, set the tree alight when it was busy with 300 diners at 9.40pm
The blaze was extinguished earlier than firefighters arrived.
A person and a lady have been handled by ambulance crews.
The fireplace brigade warned folks to maintain bare flames corresponding to candles and sparklers away from Christmas timber and decorations.