A 15-ton ball of congealed fat -- dubbed Britain's biggest ever "fatberg" -- was removed from a London sewer after a 10-day operation following complaints from local residents that their toilets would not flush.

The monstrous lump of festering food fat mixed with wet wipes -- the size of a bus -- formed in drains under a major road in Kingston, southwest London, utility firm Thames Water said Tuesday.

Had it not been removed, the deposit could have led to sewage flooding homes, streets and businesses in the leafy London suburb, Thames Water said.

"While we've removed greater volumes of fat from under central London in the past, we've never seen a single, congealed lump of lard this big clogging our sewers before," Gordon Hailwood, waste contracts supervisor for the company, said in a statement.

"Given we've got the biggest sewers and this is the biggest 'fatberg' we've encountered, we reckon it has to be the biggest such berg in British history."

"The sewer was almost completely clogged with over 15 tonnes of fat. If we hadn't discovered it in time, raw sewage could have started spurting out of manholes across the whole of Kingston."

"It was so big it damaged the sewer and repairs will take up to six weeks."

CCTV images from the sewer showed that the mound of fat had reduced the 27 by 19-inch drain to five per cent of its normal capacity.

CountyClean Environmental Services, the waste management company that removed the deposit, said it would go to good use.

"We recycle everything that we remove -- the water is extracted and the remaining fats and oils are turned into products like soap, biodiesel and fuel," a CountyClean spokesman told AFP.

"We have a very specialized piece of equipment -- called a Kroll recycler -- that we can use from the road and allows us to remove the fat without any workmen having to descend into the sewers."

Thames Water warned homes and businesses needed to change their ways when it came to the disposal of fat and wet wipes, urging people to "Bin it -- don't block it."