A fire started at 13.20 GMT at the Royal Marsden Hospital, Chelsea, London on Wednesday, pushing the authorities to evacuate 150 patients and all stuff.

Patients, some being laid on mattresses in an ambulance bay and other given refuge in a local church were relocated in other hospitals including the Royal Brompton and Chelsea and Westminster. More than 800 staff was evacuated from the hospital as surrounding streets were shut to allow the firefighters to tackle the blaze, which resulted in clouds of dark smoke billowing over southwest London.

According to Martin Gore, the hospital’s medical director, two patients were evacuated mid-way through their surgeries while a third was moved just after their procedure. However, he added that “all patients are safe,” the Reuters UK reported.

According to the London Fire Brigade, the “very big and serious fire” began in the roofspace of the newer part of the mainly Victorian hospital which was built in the late 1980s. The blaze spread fast and made the evacuation inevitable. An entire floor of the hospital was gutted.

Fire officials were investigating the cause of the fire on Thursday, after a team of 125 firefighters backed by 25 fire engines put out the fire, London Fire Brigade spokesperson Graham Howgate told reporters outside the hospitals, according to International Herald Tribune. He also added that it was too soon to speculate on what was the cause of the fire.

The hospital was built in 1851 by Dr. William Marsden, whose wife died of the disease and was the first in the world designed to cancer treatment and research into the cause of cancer.

"I want to found a hospital for the treatment of cancer, and for the study of the disease, for at the present time we know absolutely nothing about it," he said at the time. The Marsden treats about 40,000 people a year.

The fire was classified as a disaster by Peter Blake, head of radiotherapy at the hospital.

“Financially it’s a disaster. I think it will have a big effect on surgery as well,” he said.

"This fire is a terrible incident and while the distress to patients being evacuated cannot be underestimated, it is a great tribute to the staff and the emergency services that the evacuation of the entire hospital was done safely. I began my working career at the Royal Marsden Hospital and experienced first-hand the hard work that is done to tackle cancer," Ken Livingstone, the Mayor of London said according the Independent.