Terrorism law does not adequately deal with those who travel abroad to align with organisations like the so-called Islamic State, the terror watchdog has reported.

Jonathan Hall KC, the reviewer of terrorism legislation, recommended introducing a new terrorist travel offence in a report published on Thursday.

Mr Hall wrote that “long-standing difficulties continue to bedevil the prosecution of individuals who have travelled to terror zones”.

He said this “especially” applied to women “whose role may have been domestic but vital to the aims of the putative Islamic State”.

Mr Hall wrote that extra-territorial jurisdiction should be applied to child mistreatment cases – where parents take their children with them.

In her response to the report, the Home Secretary said she accepted the recommendation to consider introducing a new offence of travelling to support a proscribed organisation.

In a letter to Mr Hall, Yvette Cooper wrote: “My officials will continue to work with operational partners to assess the various elements of a potential new offence, the extent to which any gap is preventing prosecutions in practice and identify, if a new offence is taken forward, what humanitarian exemptions and other safeguards might be necessary.”

The Home Secretary said she agreed an amendment should be made to make it clearer that section 22 of the Counter-Terrorism Act 2008, which allows a Crown Court judge to authorise postcharge questioning, can be used in extradition cases.

Ms Cooper wrote that this would “support efforts to prosecute individuals, such as foreign terrorist fighters, who return to the UK”.

Mr Hall wrote that the “overwhelming motive for travel to terror zones” to date was Islamist terrorism.

He said there was “no prospect” of suspected foreign terrorist fighters being questioned by Counter Terrorism Police while in detention in countries such as Syria or Iraq, because any admissions would likely be found unreliable.

The Home Secretary wrote that she also welcomed a recommendation to establish a system of facial recognition for all arrivals at Western Jet Foil, the marina at the Port of Dover where small boat arrivals are brought after being picked up by UK vessels.

In his report, Mr Hall wrote that the “overwhelming majority” of arrivals at Western Jet Foil were “young adult men”.

He added: “In turn many of these will be from unstable countries such as Afghanistan where terrorism is endemic, and in other Schedule 7 settings would be prime candidates for possible examination.”

Mr Hall also recommended that official statistics for terrorism-related arrests should record whether the arrest relates to Islamist extremist terrorism, extreme right-wing terrorism, or other terrorism.

The Home Secretary responded that she agreed “wherever possible it is important to provide transparency on the use of counter-terrorism powers”, adding she recognised “the benefits that official statistics can provide in understanding emerging trends”.

Mr Hall said the fluctuating territories of IS in Syria and Iraq “attracted thousands of travellers”, with an estimated 5,000 coming from Europe – including 900 UK-linked individuals of national security concern of whom about 25% were killed and half have returned.

The reviewer of terrorism legislation said emerging destinations for terror activity include the Sahel in Africa, especially Mali, while post-US withdrawal Afghanistan has “resurrected fears about terrorist safe havens”.

A separate Home Office report published on Thursday found that 221 people were arrested for terrorism-related activity in the year ending December 31 2023, an increase of 32% from the 167 arrests in the previous year.

The department’s counter-terrorism disruptive powers report for 2023 found there were 244 persons in custody in Great Britain for terrorism-connected offences as of December 31 2023.

This total was comprised of 158 people (65%) in custody who held Islamist extremist views, 64 (26%) who held extreme right-wing views, and a further 22 (9%) individuals who subscribed to other ideologies.

The report also stated that during 2023, two people were deprived of British citizenship on the basis that to do so was “conducive to the public good”.