British utility company Thames Water employees have discovered what it calls the biggest "fatberg" ever recorded in the country â€” a nearly 14-tonne blob of congealed fat and baby wipes lodged in a sewer drain.

That's enough "wrongly flushed festering food fat mixed with wet wipes" to fill a double-decker bus, a company spokesperson said.

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A sewer inspector, or 'flusher,' removes a build-up of fat from a sewer wall in May 2007. (Luke MacGregor/Reuters)

Thames Water deals with fatbergs all the time, thanks to the widespread use of household oil and food fat. But few reach the mammoth size of the one found under a road in the London suburb of Kingston.

With 108,000 kilometres of sewer pipes to monitor, and fatbergs forming around even a few wipes that catch on to a corner or a wall, Thames Water says it must be constantly vigilant.

This blockage â€” built up over an estimated six months â€”was discovered after residents in nearby apartment buildings were unable to flush their toilets.

Examination found that the mound of fat had reduced the 70-centimetre by 48-centimetre sewer to just five per cent of its normal capacity.  It damaged the sewers so badly that it will take six weeks to repair them. 

The company said Tuesday it was sharing news of the massive lard lump in hopes that customers will think twice about what they dump down the drain. It also released video footage of the fatberg, filmed by a remote vehicle gliding through the sewer like an underground amusement park ride.

The company said untreated fatbergs can cause flooding and backups.

"It's very lucky we caught this one," said Craig Rance, a spokesman for Thames Water.