The blockage has been described as ” an industrial amount of solidified cement .”
Thames Water said it is 100 metres long and weighs 105 tonnes – as heavy as a blue whale.
The concreteberg is blocking three Victorian sewers in Islington and work to remove it will last at least two months.
Teams will have to manually chip away at the rock-hard mass using jackhammer pneumatic drills and high-pressure jets.
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.”
Thames Water has launched an investigation into how the concrete got into the sewer and who could be liable for removal costs.