A former British soldier who travelled to Syria to fight Islamic State (Isis) militants has said he was prompted by the murder of the aid volunteer Alan Henning.

Last month, the Observer revealed that Jamie Read was among a growing cohort of Britons fighting against Isis. He had gone to the war-torn country with another former British army soldier, James Hughes, to join a band of foreign fighters aiding the Kurdish peshmerga fighters.

Henning, a 47-year-old taxi driver, was killed by a masked militant in a video released online in October following a string of similarly publicised murders.

Read, 24, told the Sun that he believed Isis was “the biggest threat the world faces” and felt compelled to act, even though his family had concerns.

“Killing the aid worker was the final straw,” he said. “There is no justification for their executions – for putting innocent guys on their knees and doing that.

“My family were nervous and obviously worried about my well-being – we have gone into an unknown world. But I’m a firm believer that if you want to do something you have to do it and not just talk about it.”

Hughes, 26, from Reading, Berkshire, said: “I wanted to help. The situation in England is getting bad in terms of the support Isis get. The world needs to open its eyes to the threat they pose.”

According to his Facebook profile, Hughes served in Afghanistan three times and left the army this year after five years’ service. The Facebook page of Read, from Newmains, North Lanarkshire, says he trained with the French army.

The Home Office has warned against travelling to Syria and said anyone who did so was putting themselves in “considerable danger”. It said people who wanted to help should donate to charities involved in relief operations.

Meanwhile, official figures suggest more than 500 Britons have travelled to fight for Isis in Syria and Iraq.

The militant group has posted a series of videos online showing the murders of the US journalists James Foley and Steven Sotloff, the US aid worker Peter Kassig and British aid workers David Haines and Henning.

Footage seemingly showing Henning’s murder appeared on the internet just days after the UK joined US-led air strikes against the terrorists in Iraq.

Security services have also been attempting to uncover the identity of “Jihadi John”, the killer in the videos, who appears to speak with a British accent.

A Home Office spokeswoman said at the time: “The best way to help the people of these countries is to donate to registered charities that have ongoing relief operations, not by taking part in a conflict overseas, which can be an offence under both criminal and terror laws.

“People who commit, plan and support acts of terror abroad and seek to return to the UK will be prosecuted by the UK authorities. Whether a prosecution for an offence is justified in an individual case is a matter for the Crown Prosecution Service to decide after a full police investigation.”

Last month the government announced a number of new anti-terror measures designed to combat the risk of Isis radicals returning to Britain, including blocking suspected fighters’ re-entry to the UK.