The 64-metre-long mass of hardened fat, oil and wet wipes has been discovered under the streets of Sidmouth.
Fatbergs are enormous bulks of congealed waste that form when people flush waste such as fat, nappies, wet wipes and sanitary towels down their toilets.
South West Water (SWW) has said the one in Devon, which is longer than four double-decker buses, could take eight weeks to remove.
Andrew Roantree, SWW’s director of wastewater, said: “It shows how this key environment issue is not just facing the UK’s cities, but right here in our coastal towns.
“It is the largest discovered in our service history and will take our sewer team around eight weeks to dissect this monster in exceptionally challenging work conditions.”
Mr Roantree has warned the public to be more wary about what they dispose of down their plugholes.
He added: “If you keep just one New Year’s resolution this year, let it be not to pour fats, oil, or grease down the drain, or flush wet wipes down the loo.
“The consequences can be significant, including sewer flooding in your own home.
“Put your pipes on a diet and don’t feed the fatberg.”
South West Water has assured the public there is no risk to bathing waters in the seaside town, and work to remove the blockage is due to start on 4 February.
A 130-ton fatberg was discovered under Whitechapel, east London, in 2017 and took nine weeks for Thames Water to blast and hack away.
Part of the grubby mound, which was the length of two football pitches, is now in the Museum of London.