You'd think humans and animals only have the ability to accumulate fat deposits in their bodies. Apparently not. Under the streets of London, a 15-tonne lump of food fat the size of a double-decker-bus has been discovered which could only have mounted up over the years.

"If we hadn't discovered it in time, raw sewage could have started spurting out of manholes," Gordon Hailwood, Thames Water's waste contracts supervisor, said.

Residents in a well-to-do suburb in Kingston, southwest London, earlier complained on July 17 that their toilets would not flush properly, prompting utility company Thames Water to check what's going on down under the roads.

With the help of city engineers, Thames Water sent down a probe along the sewer system, and lo and behold, similar to a scientist's feeling of first discovery, the crew got the surprise of their lives when they saw the huge mass of fat deposit, now known as "fatberger" that's stuck to the top of a London sewer.

Accumulation of 15-tonne Fatberg Ball Deposits Clog London Sewer

"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," Mr Hailwood said.

The mound of fat had reduced the 70-by-48-centimetre sewer to just 5 per cent of its normal capacity.

"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 took the workers 10 greasy nights, not to mention bearing with the stink smell, just to dislodge the fatberg, which is food fat mixed with other waste, chiefly sanitary wipes, with high-pressure jets. And quite unfortunately, the huge mass has also damaged the sewer. Repairs will take up to six weeks, Mr Hailwood said.

He addressed residents to be more cautious in properly disposing their home waste.

"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.

"The wipes break down and collect on joints and then the fat congeals. Then more fat builds up. It's getting worse. More wet wipes are being used and flushed," Simon Evans, a Thames Water spokesman, said.