The company said 12,386 blockages have been cleared in Havering and 8,998 in Barking and Dagenham, treble the amount found in the whole of Oxfordshire.
To help tackle the problem, Thames Water has launched a campaign urging customers not to flush anything other than toilet paper and human waste.
The company will be offering free ‘fat traps’ to encourage customers to throw used cooking fat in the bin rather than pouring it down the sink, and billboard and bus stop posters will be appearing across the boroughs.
Formed when leftover cooking fat and wet wipes congeal into a solid mass in sewers, fatbergs block pipes and can cause sewage to back up into people’s homes and gardens. They cost Thames around £1 million a month to clear.
Thames Water head of customer field services Jerry White said: “Often people just don’t realise the consequences of putting things other than human waste and toilet paper down their toilets and drains – the term ‘flushable’ doesn’t mean biodegradable and therefore will cause a blockage.
“Having sewage flood your property or business as a result of a blockage is hugely distressing and, in many cases, avoidable. We hope this major information campaign will educate people and encourage them to ‘Bin it – don’t block it’, enabling sewers to do the job they were designed to do. To have so many sewer blockages in one borough is unacceptable.”
The company’s sewer operations manager for Havering and Barking Jon Wright said: “The sewers serve an important purpose – they are not an abyss for household rubbish.
Wright added that the team spends hours removing blockages which increase wear and tear on the pipes.
“The more fatbergs there are the more damage they cause, which inevitably results in us having to dig up roads to fix broken sewers. All of this causes disruption to our customers, and ultimately makes an impact on customer bills,” he said.