Galliard Construction, which has several workers based at the 10-storey building that is billowing huge smoke clouds across London’s skyline, said all its workers at the site have been evacuated safely.
The building has been identified as the one being refurbished to a 5-star hotel and about 86 luxury flats to a design by Norman Foster's architectural firm.
Nearly 30 firefighters, more than 70 fire personnel and 15 fire engines were sent to the site, said a London Fire Brigade spokesman. The fire took nearly a quarter of the 100sq m (1,076sq ft) of the roof of the building, he added.
Earlier, thick plumes of smoke could be seen rising above the Marconi House which is at the junction of the Strand and Aldwych.
There were no reports of people trapped or injured and the cause of the fire is still unknown.
The London Ambulance Service has sent five specialist hazardous area response team units and paramedics to the location as a preventative measure.