As many as 70 high-threat jihadists have returned to the UK, a Home Office official has warned. The claim comes as the UK Border Force says its lack of powers is crippling security.

Scott Wilson, coordinator of the Home Office Protect and Prepare counter-terrorism program, told the Security and Counter Terror Expo conference in London that up to a fifth of the 350 returned jihadists could carry out deadly attacks in the UK.

He said support for Islamic State (IS, formerly ISIS/ISIL) in the UK had outstripped that of Al-Qaeda, warning that while he could not say “where it is going to go” the new threat would endure “for a long, long time.”

Wilson’s comment comes as the UK Border Force warned it lacks the necessary powers to deal with jihadists entering the UK, raising fear that so-called “clean skins” – those of whom the security services have no prior knowledge – present a serious danger.

Their complaint hinges on the fact that unlike police officers they cannot use the Terrorism Act 2000, which allows officials to “stop, question and detain” any person they believe is “concerned in the commission, preparation or instigation of acts of terrorism.”

An anonymous Home Office whistleblower told the Telegraph: “They have conferred no specific powers under Schedule 7 of the Terrorism Act on immigration officers in arrivals halls. It would be unlawful for them to detain a Brit.”

At present, the only time a Briton can be detained by the Border Force is when their name is already on a “warnings index,” which means that those who have not been “flagged” cannot be stopped.

David Burrowes, a Tory member of the Home Affairs Select Committee, told the Telegraph: “You would expect border guards to have the force of the law, which lead to arrest and search and other powers that you would expect of an ordinary police officer.”

“We are talking about the risk from terrorism and we need to give them the tools necessary to deal with that,” had added.

A Home Office spokesman said the UK Border Force had all the necessary powers including the capacity to refer suspicious persons to the police.