People pouring concrete into sewers has led to a “concreteberg” forming in central London that weighs 105 tonnes, as heavy as a blue whale.

The 100-metre-long mass is blocking three Victorian-era sewers in the heart of the capital. Thames Water’s operations manager, Alex Saunders, said it was the largest mass of concrete the company had seen, and could take two months to remove at a cost of at least several hundred thousand pounds.

The work to tear it apart will take place at the junction of Goswell Road and Hall Street in Islington and is likely to cause traffic disruption.

Saunders said: “Normally blockages are caused by fat, oil and wet wipes building up in the sewer but unfortunately in this case it’s rock-hard concrete. It’s in there and set to the Victorian brickwork, so we need to chip away at it to get it removed.

“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. It’s very frustrating and takes a great amount of time and effort to resolve. 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.”

Sewers are more commonly afflicted by fatbergs, congealed masses formed by a mix of non-biodegradable solid matter, such as wet wipes. They have become a problem in England, the result of an ageing Victorian sewer network and a rise in the usage of disposable or flushable cloths.

The concreteberg is believed to have been caused by a construction company pouring surplus cement down a drain. A range of cutting tools, including jackhammer pneumatic drills and high-pressure jets, will be needed to break it up.

Thames Water spends £18m every year clearing blockages from its sewers. The company has pledged to invest heavily to improve the network and increase monitoring as part of its business plan for 2020-25, using up to 200,000 new sewer depth digital monitors.

Thames Water said it had launched an investigation to identify the culprits and planned to sue them to recover its costs.

In 2018, Thames Water was called to clear 42,000 blockages caused by fat and non-biodegradable matter, a 6% rise on the year before. In 2013, a fatberg the size of a bus was found in a sewer in Kingston-upon-Thames.