Smoke was spotted billowing out of the former Midland Grand Hotel - also known as St Pancras Chambers - on Euston Road before midday on Monday 23 July.
Eight fire engines and 45 fire-fighters were called to the scene and the blaze was under control by 1.15pm.
According to an LFB spokesman, part of the ground floor has been damaged, but the cause of the fire is still unknown.
At one point police and fire crews feared the blaze would engulf nearby gas canisters, triggering evacuation of the British Library and Camden Town Hall.
There were no reported casualties, but Euston Road was shut between Hampstead Road and Tottenham Court Road, causing traffic chaos.
RHWL with Richard Griffiths Architects is currently transforming the 32,300m2, Grade I-listed building - which has stood empty for 20 years - into a new five-star, 245-bed hotel.
The scheme, for the Manhattan Loft Corporation, retains the external appearance of the original building, while adding a 'sympathetically designed' hotel extension at the rear. Work is due for completion in 2008.