Giant balls of congealed fat under Sidmouth's streets pose a serious risk of more foul flooding from the town's sewers, South West Water has warned.
The firm's top boss says a build-up of disgusting deposits, known as 'fatbergs', are being caused by residents and businesses who pour grease and oil down sinks and drains.
More than £1million is spent by the company removing a shocking 21 double-decker buses-worth of the toxic clogs across its network each year.
He wrote to the firm's chief executive, Chris Loughlin, to express his 'horror' when sewage seeped on to the seafront as 300 young scouts marched along The Esplanade as part of a St George's Day parade on April 27.
Mr Swire said the 'utterly disgusting' floods were 'wholly unacceptable' and 'must never happen again'.
Mr Loughlin said the incident was caused by a combination of heavy rainfall 'exacerbated' by a build-up of waste fat, cooking oil and grease, adding: "Thousands of litres of these are poured down sinks by householders and commercial operators who are unaware of the problems it may cause.
"They cool quickly to form a solid mass which can build up in pipes and block the sewerage system. Most sewer blockages can be avoided if drains and sewers are only used for their intended purpose instead of general waste disposal.
"As a company, we spend more than a million pounds a year to remove up to 3,000 tonnes of solidified fat from the sewers - the equivalent volume to 21 double-decker buses.
"Additionally, fat in sewers attracts vermin and if a blockage results in pollution of rivers and streams, it can also have a damaging impact on the environment."
He said advice is issued to domestic and business customers.
"We are currently investigating a number of locations in Sidmouth where we hope to carry out some sewer cleaning to reduce the risk of flooding and the seafront area will be visited as a priority," added Mr Loughlin.
He apologised for any distress caused to those attending the parade - which was a huge success despite the downpours.
Mr Swire told the Herald he was 'encouraged and grateful' SWW has acted, adding: "It is clear that businesses and householders also have an important part to play in ensuring that they do not contribute to the blocking of the town's drains."