The Royal Marsden Hospital is probably the best-known cancer hospital in the UK, and one of the most famous in the world.
Every year, 40,000 patients come through its doors, while in the background, hundreds of researchers work on the drugs and treatments which will save patients in future years.
The Marsden was the first hospital in the world to be dedicated to cancer, founded in 1851 by a doctor who watched his own wife die from the disease. It moved to its current site on the Fulham Road in 1862.
It is the country's only designated Biomedical Research Centre for Cancer, with a £10 million annual grant, and remains one of the most important cancer hospitals in the world.
There were only six "centres of excellence" for cancer in the UK named in the NHS Plan in 2000 - and the Royal Marsden was one of them.
The largest comprehensive cancer centre in Europe, the Royal Marsden Foundation Trust is also one of the highest rated in England, with an "excellent" score in the Healthcare Commission's Annual Health Check for each of the last two years - the only trust to achieve this.
Breakthroughs shared by the Marsden and the neighbouring Institute of Cancer Research include the first discovery of the carcinogenic chemicals in tobacco smoke in the 1920s, the first chemotherapy drugs in the 1950s, and new combinations in the 1980s and 1990s, including drugs giving a 96% cure rate for testicular cancer.
In 1991 the Marsden became the first NHS hospital to be awarded the Queen's Award for Technology for its work on drug development.
Just in the past year, researchers at the Marsden worked on drug treatments for breast and prostate cancer, and advances in MRI scanning to help diagnose the disease.
Famous patients have included Beatles' guitarist George Harrison, Rolling Stones' drummer Charlie Watts and writer and broadcaster John Diamond.
Jonathan Kipling, chief executive of the Institute for Cancer Research, said: "It's a tragedy, as anything more than short-term disruption to the delivery of cancer treatments will have obvious consequences for their patients.
"We can only hope that the damage is not extensive."
He described the hospital as the "leading centre" in Europe for cancer research, particularly in the development of new drugs.
"In terms of published research in major scientific journals, it leads both the UK and Europe,and there are only a few centres in the US which do more."
Professor Karol Sikora, an honorary cancer consultant at Hammersmith Hospital, estimated that 30% of British oncologists had spent some time at the Marsden.
He described it as a "world-class" centre for research.
"It's one of the few leading hospitals specialising in cancer - there are only three in the UK - in terms of both the quality of its treatment and the quality of its research.
"It's very much a haven of education of British cancer."
Professor Sikora said plans would be in place to transfer patients to other hospitals in the London region.
"Hopefully there would not be too many patients within the hospital during the Christmas and New Year period, so they could be accommodated fairly easily elsewhere in London.
"In addition, the hospital has another site in Sutton which could be used to treat some patients."
He said that interruptions of a few days in courses of cancer treatment were unlikely to be damaging.
"For most cancer treatments the timing is not absolutely critical," he said.
"There are very few situations where a gap of a few days, or even a week is going to make a huge difference.
"The longer term problem comes if the damage to the roof means that none of the wards and rooms underneath can be used."