A 15-tonne ball of congealed food fat, dubbed Britain's biggest ever "fatberg", has been dredged up from a London sewer.

It took 10 days to remove the double-decker-bus-sized lump of food fat mixed with other waste, chiefly sanitary wipes, from under a major road in Kingston, southwest London, according to Utility company Thames Water.

Thames Water says it is the biggest lump of fat ever removed from the drains.

Left there much longer, it could have caused raw sewage to start flooding homes, streets and businesses throughout the area.

Local residents in the well-to-do suburb had already complained that their toilets would not flush.

Images from inside the sewer show that the rotting mound of fat had reduced the drain to 5 percent of its normal capacity.

The Thames Water website features a YouTube video showing the fat caking London's sewer walls.

"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," said Thames Water's waste contracts supervisor Gordon Hailwood.

"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."

Mr Hailwood says the huge mass damaged the sewer and repairs will take up to six weeks.

He wants residents to be more vigilant about their waste disposal habits.

"Homes and businesses need to change their ways, when it comes to fat and wipes, please remember: 'Bin it – don't block it,'" he said.

While the risk of stomach-churning sewage belching up through people's toilets might have been averted, Londoners could soon be washing with it anyway.

CountyClean Environmental Services, the waste management company that removed the deposit, says the fatberg will be put to good use.

"We recycle everything that we remove," a CountyClean spokesman said.

"The water is extracted and the remaining fats and oils are turned into products like soap, biodiesel and fuel."

AFP