Flames and thick black smoke billows above the new Ibis hotel in Ealing, west London
The fire broke out at the Ibis hotel which is still under construction.
London Fire Brigade said it received the first of more than 40 calls from members of the public about the fire at 11.30am.
Ten fire engines and 72 firefighters and officers were sent to tackle the blaze on the busy Uxbridge Road.
Images and video footage show plumes of smoke pouring from the roof of the building.
Firefighters raced to the new hotel in Ealing
Fire crews from Ealing, Acton, Southall, Chiswick, Park Royal, Wembley and surrounding fire stations are attending the blaze.
The cause of the fire is not known at this stage.
Policesaid Uxbridge road was expected to be closed for at least 12 hours.
Ealing's police and fires stations are both based on Uxbridge roads within a few yards of the scene of the blaze.