Britons who join the ranks of ISIS face up to ten years in prison under a new law aimed at creating global no-go zones for extremists. More than 900 Britons have travelled to Syria but authorities have struggled to convict returning fighters because of the difficulty of securing evidence. The new law is designed to deter extremists from travelling to designated warzones and to prosecute them on their return if they flout the law. The new legislation – which was signed into law on Tuesday - comes as Britain seeks to outsource the trials of two of its most notorious extremists - Alexanda Kotey and El Shafee Elsheikh – who are accused of being part of the ISIS assassination squad. They have been implicated in the murders of three US and two British citizens but UK prosecutors said they were unlikely to secure a successful conviction because of their treatment while being held in custody. The men, who have been stripped of their British nationality, are currently being held by western-backed Kurdish forces in northern Syria but could go on trial as early as the summer in the United States, according to media reports. The new law makes it illegal for a UK national or resident to enter or remain in an area designated by the government and then agreed by parliament. It could include entire countries, said the UK’s home office, which said the first areas could be in place by April this year. It also does not apply retrospectively for anyone who travelled to fight in Iraq and Syria. “The stable door has in part closed since IS do not occupy much territory any more,” said Alex Carlile, a former independent reviewer of terrorism legislation and senior associate fellow at the Royal United Services Institute. “But it’s to be expected that so-called Islamic State is likely to revert to terrorism having lost their territory and will hold small pockets. I don’t suppose there will be many cases where it will be used but those will be important cases.” The legislation does not apply to people travelling to those countries for funerals, to care for terminally-ill relatives, or for certain professions, including UN and humanitarian workers, government staff and journalists. Denmark and Australia have a similar offence which has been used to prosecute its nationals returning from Iraq and Syria. An impact assessment by the government said it anticipated “low” use of the legislation. Theresa May, the prime minister, announced a review of the government’s counter-terrorism strategy after the London Bridge attacks when a group of extremists mowed down pedestrians in a van before embarking on a stabbing spree which left eight dead. The attack was one of five targeting the UK in 2017 that left 36 people dead, prompting demands for stricter rules to target British extremists, who have travelled abroad to secure training and battle experience, and home-grown terrorists. The legislation also tightens the rules around hate preachers who manage to remain on the right side of the law despite inspiring their adherents to join banned groups. Anjem Choudary, the legally-trained former head of the banned organisation Al Muhajiroun, avoided prosecution for years despite his propaganda motivating at least 100 people to pursue terrorism. He was finally jailed for five-and-a-half years in 2016 after urging support for ISIS. The legislation also updates counter-terrorism laws for the technological age, making it an offence to view or stream terrorist content. Previous laws required defendants to download the material before it became an offence. Security minister Ben Wallace said the law would ensure that security officials had “the powers they need to tackle the evolving threat posed to the UK by terrorism and hostile state activity”.