The fire started on the fourth floor and destroyed much of the roof

Operations had to be halted and patients and staff were led to safety as fire broke out at the leading cancer treatment centre.

At its height, up to 125 firefighters were at the scene in Chelsea.

Only four people needed treatment for the effects of smoke from the fire which started on the fourth floor.

It is understood investigations will centre on this area of the west London hospital where construction work was taking place.

It became apparent it was not a little fire, but they kept it quite calm Colin Bishop, patient

David Brown, assistant commissioner of the London Fire Brigade, said: "A plant room has been involved in the fire but it is too early to say exactly where the fire started."

Five operating theatres and two wards have been badly affected while much of the roof was destroyed by the fire which broke out shortly before 1330 GMT on Thursday.

About 800 staff and up to 160 patients and out-patients were moved to safety from the hospital, including two anaesthetised patients undergoing surgery at the time.

The pair were safely taken off their ventilators and moved to a neighbouring hospital.

More than 120 firefighters tackled the blaze

Several patients had to be laid on mattresses in an ambulance area on Dovehouse Street while nursing staff led others away from the scene wrapped in blankets.

A number were taken to St Paul's Church in Onslow Square or transferred to the Royal Brompton Hospital.

One patient, Carole Williams, was in bed when the alarm sounded.

"I could smell smoke, and we were told to get out."

The 55-year-old mother-of-four added that "there was no panic whatsoever".

'Surreal' feeling

Another patient, Colin Bishop, was waiting for his first session of chemotherapy treatment for throat cancer.

"There was smoke and it was coming through the ward. It became apparent it was not a little fire, but they kept it quite calm," he said.

Dr Aled Jones, a surgical doctor, was led out of the hospital in a break between operations.

"We did think it could have been a false alarm, but the message spread quickly around the hospital and we could smell the smoke," he said.

"It was surreal. It didn't feel like it was really happening," he added.

The Royal Marsden was the first hospital in the world dedicated to cancer treatment and research, seeing more than 40,000 patients from the UK and abroad every year.