It was a rainy evening in the City of London, the sparkling lights of the financial district shining like diamonds on the wet pavement. Two young men, clad in high-vis vests and hoods to conceal their faces, made their way through deserted streets, evading the watchful eyes of police and private security cameras.

Their target was a maintenance hole on Lime Street, where they hoped to find the perfect opportunity to strike. Reaching the location, one of them carefully removed the cover, revealing a bundle of black cables. With calculated precision, he began to hack away at the wires, determined to disrupt the city's internet infrastructure.

Hours later, an email circulated among news desks, announcing that hundreds of insurers had been cut off from the internet as part of climate-motivated sabotage. The group behind the operation, Shut the System (STS), claimed responsibility for the action, stating it was a "new phase" in their campaign to shut down key actors in the fossil fuel economy.

STS is not the first group to take clandestine direct action against fossil fuel targets. In 2022, unknown activists targeted a pipeline under construction, cutting holes in the pipe and severing hydraulic cables on a construction vehicle. This month, another group claimed responsibility for drilling holes in the tires of over 100 SUVs parked at Land Rover dealerships in Cornwall.

The Tyre Extinguishers, a campaign group urging people to take autonomous clandestine action against SUVs by deflating their tires, have targeted hundreds of vehicles through activists heeding their call. But with climate breakdown worsening and fossil fuel emissions showing no signs of peaking, some activists are turning to more extreme measures.

"The actual number of people who are committed to risk jail time to do this are pretty small in number," said an activist from STS, speaking over the Signal encrypted messaging service. "New laws further criminalizing disruptive protests have made traditional, accountable methods of activism increasingly unsustainable, and a clandestine approach increasingly attractive."

The activist pointed to the case of activists from Just Stop Oil (JSO) who received sentences of four and five years – reduced last week after an appeal – for organizing road blocks on the M25. "If you want to do anything that is disruptive, the penalty is pretty massive now, and so these draconian laws mean it is hard to get very much pressure...by following the kind of things that [Extinction Rebellion] and JSO have done in the past, because people will be arrested and put away for a long time."

Andreas Malm, an associate professor of human ecology at Lund University, notes that France has been the most widely used tactic of sabotage among climate activists. "France really is the one case in recent years...where you've had a radical mass movement that has actually been quite successful—and this is the only movement that has also deployed sabotage consistently as a tactic."

Malm attributes the success of French activism to its willingness to engage in militant action, rather than sticking to non-violent disruptive protest. "One mistake made by the offshoots of XR [such as Just Stop Oil]—they started escalating a little bit and doing slightly more radical stuff…while still sticking to the protocol to wait until the cops come and arrest them," he said.

"If you want to actually escalate and do real material damage to fossil fuel property, you cannot stick to this idea. You have to do this without offering yourself as a kind of virtue sacrifice." The STS activist who spoke to the Guardian did not see their group's actions as more extreme than those already carried out by other groups.

"The only difference is that they stayed around to be arrested," he said, highlighting the stark contrast between the activists' willingness to engage in militant action and their predecessors' focus on non-violent protest.