A 64-metre "fatberg", a mass of congealed waste has been found in a sewer under the British seaside town of Sidmouth, in the southwestern county of Devon, a water company says.

"Devon's largest fatberg has been discovered in Sidmouth," South West Water said on Twitter on Tuesday.

"It's a whopping 64 metres long; that's over six double-decker buses back to back," it said, posting two photographs of the grey "fatberg" filling most of the brick and concrete sewer.

South West Water urged local residents to "think sink" and "love your loo".

"Don't pour cooking oil, fat and grease down the sink [and] only flush the (hash)3Ps - pee, paper and poo," it tweeted.

Andrew Roantree, South West Water's director of wastewater, said he expected engineers to need about eight weeks to "dissect this monster in exceptionally challenging work conditions".

They will start work on February 4, wearing breathing apparatus to remove the fatberg manually and with "special sewer jetting equipment", the company said.

A much larger fatberg, stretching about 250 metres, blocked a sewer in east London's Whitechapel district in 2017. Part of it was exhibited at the Museum of London last year.

Roantree said the Sidmouth fatberg showed "how this key environmental issue is not just facing the UK's cities, but right here in our coastal towns".

"Thankfully, it has been identified in good time with no risk to bathing waters," he added.

Sidmouth is a holiday resort known for its sandy beaches.