The work at the junction of Goswell Road and Hall Street, Islington is likely to cause traffic disruption to motorists for at least the next two months as teams manually chip away at the rock-hard mass, the biggest the company has ever seen. The subterranean blockage is thought to be at least 100 metres long and weigh 105 tonnes – as heavy as a blue whale.

It will cost at least several hundred thousand pounds to clear using a range of cutting tools, including jackhammer pneumatic drills and high-pressure jets. This is money which could be spent on investing in the network and helping customers in vulnerable circumstances.

Residents have been informed about preparatory work which will start next week. Tankers are also on standby to pump out waste 24 hours a day to protect the environment, and ensure nearby properties and businesses are not flooded with sewage caused by the blockage.

Alex Saunders, Thames Water operations manager, 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.”

Every year Thames Water, serving 15 million people across London and the Thames Valley, spends £18 million 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. It has also proposed to reduce pollutions by 30 per cent.

An investigation into how the concrete got into the sewer and to recover costs is underway.