A record-breaking “concreteberg” as heavy as a blue whale has appeared in a London sewer.

The blockage, 100 metres long and weighing 105 tonnes, is the largest that Thames Water has ever seen. The firm blamed workmen pouring concrete down drains for it.

It said it believed it will take at least two months to remove with specialist equipment including pneumatic drills and high-pressure jets, at a cost of several hundred thousand pounds or more.

The solidified concrete, found during a routine check under Hall Street in Islington, is denser than a “fatberg” of fat, oil and wet wipes, and has set to the walls of the Victorian-era sewer.

“This is not the first time damage has been caused by people pouring concrete into our sewers but it’s certainly the worst we’ve seen," said Alex Saunders, Thames Water operations manager. “It’s very frustrating and takes a great amount of time and effort to resolve.

UK fatbergs in pictures Show all 7 1 /7 UK fatbergs in pictures UK fatbergs in pictures Sidmouth's 'monster' fatberg The blockage is 64m long AP UK fatbergs in pictures 64m fatberg in Devon This photo was released by South West Water AP UK fatbergs in pictures Workers tackle fatberg A fatberg is a mass of hardened fat, oil and baby wipes AP UK fatbergs in pictures On display A part of the Whitechapel fatberg on display in the Museum of London Museum of London UK fatbergs in pictures 130-tonne Whitechapel fatberg Authorities are tackling a huge fat blockage in the capital’s sewage system PA UK fatbergs in pictures Fatbergs by the numbers Data compiled by statistics agency Statista UK fatbergs in pictures Clogged mass A fatberg beneath Whitehall Thames Water

“We’re now doing everything we can to deal with it as quickly as possible, making sure our customers don’t have to suffer because of this mindless abuse of our network.”

When work begins next week, tankers will be on standby to pump out any backed-up sewage blocked by the concrete, Thames Water said.

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Blockages cost the company around £18m each year.