More than 40 firefighters were called to the blaze which destroyed 50 per cent of the first floor and 10 per cent of the second floor of the four-storey building.
They quickly brought the fire under control and rescued a woman from the top floor of the building in Holland Park. She was brought down on a turntable ladder and taken to the Chelsea and Westminster Hospital suffering from smoke inhalation and shock, said a spokesman for London Fire Brigade.
One firefighter was also taken to the hospital with a back injury.
Mr Branson, who drove to the building from the Midlands where he had been watching the World Superbike Championship at Donington Park, said there had been a lot of expensive equipment in the office.
"My own particular office and all my files and paperwork and everything has completely gone and there is fairly extensive damage," he said.
"Years of paperwork and all our current paperwork and letters have gone."
But in typically relaxed form, the Virgin tycoon said he was sure his secretary would be relieved that there would be less paperwork for the next couple of weeks.
The cause of the fire was not immediately apparent but Mr Branson said he was certain it was not deliberate.
"We do not suspect foul play and I would most likely think it would be an electrical fault but we don't know yet," he said.
His own office suite had received the worst damage although the rest of the building was full of smoke, said Mr Branson.Reuse content