Fire investigators are trying to work out how a massive blaze started at one of the world's leading cancer hospitals.

Dozens of patients from the Royal Marsden were forced to flee when the fire started.

Firefighters worked through the night to ensure the blaze was completely out and were expected to hand back control of the hospital in Fulham Road, west London, later.

Hospital bosses hoped to re-open a section of the building for patients and staff to ensure continuity of care but most of the site will remain closed.

Prime Minister Gordon Brown has is visited some of the patients who were evacuated from the Royal Marsden to the nearby Royal Brompton.

He said he had "nothing but the fullest of praise" for staff and emergency services who helped move the patients safely from the blaze.

Mr Brown, speaking after meeting patients, nurses and doctors, pledged the Government would do "everything in our power" to get the world renowned cancer hospital "up and running as quickly as possible."

He said: "The rebuilding work will start immediately."

Investigators from the fire service will seek to establish how the blaze started amid rumours building work could have been involved.

The hospital, which is Europe's largest comprehensive cancer centre, has confirmed a number of works were being carried out but said it was far too early to say if there was any link.

It said in a statement: "Investigations into the cause of the fire and its starting point are still ongoing and there will be no further comment until that is complete."

London Fire Brigade has already said there has so far been nothing to suggest foul play or that the fire was anything other than accidental.

Assistant commissioner David Brown said: "A plant room has been involved in the fire but it is too early to say exactly where the fire started."

Hundreds of cancer patients and staff had to be evacuated after the fire broke out and 125 firefighters and 25 fire engines were sent to tackle the flames.

Ambulances were lined up in the street as police officers, hospital staff and paramedics rushed to empty the building as the scale of the emergency became clear.

Operations on two patients who were in surgery at the time had to be cut short and six people in intensive care were among the 79 in-patients moved to other hospitals. Around 800 staff were also evacuated.

Hospital chief executive Cally Palmer said: "We're still trying to assess the extent of the damage the fire has caused but it has mainly affected one part of the building.

"Five operating theatres and at least two wards are very badly affected.

"I am hoping we can get back to full operations sooner rather than later."

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