A fire tore through the top floor of a world-leading cancer hospital in London on Wednesday, forcing the evacuation of all staff and patients, including some who were already undergoing operations.

The top floor of the Royal Marsden Hospital in southwest London was gutted as 79 inpatients and dozens more outpatients were evacuated in near-freezing weather to neighbouring hospitals and churches.

More than 800 staff, some still dressed in surgical scrubs, were evacuated from the hospital as surrounding streets were shut to allow firefighters to tackle the blaze, which sent clouds of dark smoke billowing over southwest London.

Two firefighters and a patient were taken to another hospital suffering smoke inhalation. No other patients were injured.

Senior hospital officials said the fire, which started just after 1:15pm in a fourth floor plant room, had compromised operations in two key operating theatres and recovery wards had been badly damaged.

Addressing reporters at the scene, the hospital's chief executive, Cally Palmer, said staff were working hard to help maintain services and operations.

But she said facilities had been damaged and future treatment of patients could be hampered.

"The theatres and at least two wards are very badly affected," she said. "Certainly a large proportion of... our operating capacity has been compromised."

Patient numbers overall were lower than normal because many had gone home for Christmas.

The hospital's medical director, Martin Gore, said two patients were evacuated mid-way through their operations while a third was moved just after their procedure.

"Some of the operations were partly interrupted but all patients are safe," he said.

A London Fire Brigade spokesman described the blaze as "a very big and serious fire".

He said 25 fire engines and 125 firefighters had tackled the blaze. "It is one of the larger fires we have had to deal with," he added.

The hospital was founded in 1851 and was the first in the world dedicated to cancer treatment and research into the causes of cancer.