Thames Water has found a 15 tonne bus-sized lump of rotting food and sanitary wipes in the drains beneath a road in Kingston, South West London.
The congealed mushy deposit, dubbed a 'fatberg' by the authority, is thought to be the largest ever found in Britain.
The blockage, which if left untreated could have led to sewage flooding homes, streets and businesses, was discovered after residents in nearby buildings complained that they couldn’t flush their toilets. The water authority subsequently discovered the 'fatberg' through CCTV investigation.
Gordon Hailwood, waste contracts supervisor for 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.
“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."
The water authority also issued a plea for householders to avoid disposing of fat and wipes through their drains. Such was the density of the massive 'fatberg' it had reduced the 70x48cm sewer to just five per cent of its normal capacity.
Thames Water will begin repairs to 20 metres of damaged pipe on Monday August 5 and work is expected to take up to six weeks to complete.