Only one suspect is being held under the UK’s highly restrictive Terrorism Prevention and Investigation Measure (TPIM) program, despite a heightened national alert and fears of extremist attacks.

The TPIM program, which came into effect in 2012, can include intense restrictions on suspects such as curfews and electronic tagging.

Former senior counterterrorism officer Chris Phillips told the Daily Mail that just one person being held under TPIM contrasts starkly with the fact that up to 2,000 terror suspects are known to police.

He said, however, that the sheer number of suspects makes the risk hard to manage.

“How on earth you could ever monitor 2,000 people, let alone the fact that the number that we have got now has probably increased,” he said.

Lord Carlile, an independent reviewer of UK terror laws told the Daily Mail: “It is surprising and worrying that we are down to just one Tpim given the situation appertaining all over Europe.

“We know that there is a severe risk of terror attack. I hope that the Government is examining the possibility of increasing the use of Tpims or toughening them up...it is absolutely essential that the authorities should have the powers they need.”

The UK terror threat level stands at “severe,” the second highest category available, which means an attack is thought highly likely.

On Wednesday, the Home Office told religious leaders that churches must review their security to deter terrorist attacks.

It follows Tuesday’s attack on a Normandy church, in which a Catholic priest was reportedly beheaded in front of his congregation by two self-proclaimed Islamic State (IS, formerly ISIS/ISIL) “soldiers.”

However, calls for armed security personnel outside churches were strongly rejected by religious leaders, who believe the sanctuaries should remain public and accessible venues for all.