Islamist extremists currently present the biggest terror threat to the UK, one of the country's most senior police officers has said.

Speaking at Scotland Yard, Deputy Assistant Commissioner Vicki Evans, the senior national co-ordinator for Counter Terrorism Policing, said her unit’s biggest caseload over the last 12 months came from radicalised Muslims – as opposed to the threat from the far right. 

She said that of three intended terrorist attacks officers had thwarted in the late-stage of planning in the past year, two were Islamist plots and one was extreme-right wing.

The national terrorism threat level is “substantial”, meaning an attempted attack in the UK is likely.

Detectives had not expected the current “volume and tempo” of terrorist activity, she added at the briefing.

Evans said the fall of President Bashar al-Assad in Syria had prompted concern among counter-terrorism officers, because groups like Isis "capitalise on chaos.

"History tells us that instability creates a space of extremism, violence and acts of terror,” she said.

Evans warned that the Metropolitan Police was facing a continuing threat from “self-initiated terrorists” who are radicalised online.

She said that young people were accessing “horrific” content on the internet, including terrorist material, which is promoting a fascination with violent acts.

She described how a “conveyor belt of young people” were viewing racist, violent and misogynistic content, including extreme pornography, material related to school massacres, and “incel” culture.

The search histories which the Metropolitan Police are reviewing show that this “pick and mix of horror” is available “readily and easily online”.

Speaking to reporters, she said: "These sort of grotesque fascinations with violence and harmful views that we're seeing are increasingly common.”

Since 2017, police and security services have thwarted 43 late-stage terror plots, with some of these "goal line saves", Evans said.

Speaking to the BBC, she recommended that a "whole-system approach" was required to combat the problem, and that technology companies had a duty to assist counter-terror police "with what access is available to that sort of material.

“We have some really deep, dark hot spots - some pockets where we cannot leave the activity and the groups unattended, and we need to continue to maintain our focus on them to keep the threat at bay,” she said.