Donald Trump “handed Isis a propaganda victory” that could bolster its efforts to radicalise people in the UK when he announced the withdrawal of US troops from Syria, Britain’s counterterror police chief has said.

Although the group’s online propaganda operation had been damaged by losses on the battlefield, it was still able to inspire followers to commit attack, Neil Basu warned.

“The radicalisation of vulnerable people, particularly lone actors and very suggestible, young, malleable or potentially mentally ill people, is still a great threat,” the Metropolitan Police assistant commissioner told The Independent.

“You hand Isis a propaganda victory if you say ‘we’re going to withdraw’, which they took as a sign they were able to say ‘we’ve won’. That can’t be good for us [in Britain. I would hate for anybody to be complacent enough to think that threat has gone.”

Counterterror police in the UK are currently running a record 700 investigations, with 14 Islamist and four far-right terror plots foiled since the Westminster attack in March 2017.

While the majority of plots have come from homegrown extremists, rather than returned foreign fighters, police are on the lookout for jihadis trying to re-enter the country.

According to a Home Office estimate, up to 300 British Isis recruits could still be alive in Syria and Iraq.

Several are in the custody of the Syrian Democratic Forces and with the government seeking to strip British citizenship of all dual nationals who joined Isis, Mr Basu said there is no “legal pathway” to repatriate them for trial.

Syrian Democratic Forces take Raqqa from Isis – in pictures Show all 8 1 /8 Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces fighters celebrate victory in Raqqa atop of military vehicles REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Fighters of Syrian Democratic Forces march past destroyed buildings as they celebrate victory REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Fighters of Syrian Democratic Forces gesture the "V" sign in Raqqa REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Fighters of Syrian Democratic Forces evacuate a civilian from the stadium REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures A civilian prays after she was rescued by fighters of Syrian Democratic Forces from the stadium REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces fighters ride atop of military vehicles as they celebrate victory in Raqqa REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces fighters celebrate victory in Raqqa REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures A fighter of Syrian Democratic Forces celebrates in Raqqa Reuters

“We’ve only seen a trickle of people return – most of these people don’t want to come back here,” he added.

“I’m concerned that people will either get back without our knowledge, because that’s entirely possible, or that we will not have a case we can prosecute.”

Although Isis has lost virtually all of its 2014 “caliphate” in Iraq and Syria, militants are still battling in isolated pockets and are active in Egypt, Afghanistan and other countries.

“We have defeated Isis in Syria, my only reason for being there during the Trump presidency,” Mr Trump tweeted in December.

The announcement sparked a spate of resignations in the Trump administration, and international pressure for the move to be delayed.

Following predictions that Isis would be emboldened to launch counterattacks, the group has carried out a series of bombings including one that killed US soldiers.

Propaganda channels on the encrypted Telegram app continue to post regular updates on Isis’s military operations, terror attacks and assassinations to thousands of followers around the world.

Mr Basu said although Isis’s influence operation had been “tremendously affected” by territorial losses, the scale of its propaganda machine meant that the struggle to remove material from the internet continues.

Terror police appeal for public help to thwart plots

“A lot of it still remains out there and accessible,” he added. “You still have encrypted groups where people can talk to each other.”

The Counter-Terrorism and Border Security Bill, which was approved by parliament on Tuesday, will make it illegal to enter “designated areas” abroad without a reasonable excuse.

Mr Basu said the controversial offence would make it easier to prosecute Isis fighters, whose activities have previously been difficult to prove.

He also gave his backing to a review of the Prevent counter-extremism programme that was announced as part of the bill, and a law against viewing terrorist material online.

Mr Basu said the law, which sparked human rights concerns, would be used with “discretion” and was necessary to reflect the fact terrorists now rarely download extremist material, an action that can be prosecuted.

“We’re talking about people who are a serious threat here,” he added. “I don’t think we’re going to see an explosion in arrests and charges.”