The latest western hostage to be threatened with murder by Islamic State militants is a taxi driver from north-west England who fell into the group's hands after joining a group of Muslim friends on an aid convoy to Syria.

Alan Henning, described by friends as "a big man with a big heart", volunteered to join the humanitarian mission last Christmas after helping to raise funds to purchase the medical equipment that the convoy was taking into the country.

The vehicles were halted by masked gunmen after crossing the Turkish border and Henning, 47, was separated from his friends, according to accounts they gave to journalists after they were released by the group and able to return to Britain.

Since then, they and Henning's family have remained silent about his plight, at the suggestion of the UK Foreign Office (FCO), while behind-the-scenes attempts were made to negotiate his release.

On Sunday, after Henning was shown and named in the latest video posted online by Islamic State, the FCO withdrew a request to the media to avoid identifying him, and distributed a photograph taken on a trip to Syria.

The group's video, entitled A Message to the Allies of America, first shows David Haines, the Scottish aid worker, being forced to deliver a speech in which he blames his death upon David Cameron and Britain's relationship with the United States. It then depicts Haines' murder by beheading.

The killer's voice and his distinctive London accent suggest that he is the man who was previously responsible for the video-taped murders of American journalists James Foley and Steven Sotloff. At the end of the two-and-a-half minute film, the killer is shown holding a distraught Henning by the collar of the orange jumpsuit in which he is dressed. Indicating towards his captive with his knife, the killer says: "If you, Cameron, persist in fighting the Islamic State, then you, like your master Obama, will have the blood of your people on your hands."

His abduction is said to have distressed members of the group of amateur aid workers who organised the convoy, and angered some members of the Muslim communities in which they live, with accusations that a non-Muslim should never have been allowed to join the mission because of the risk. It was his second trip to Syria – a first visit nine months earlier had been trouble-free.

Henning was among a group of volunteers who raised money to purchase medical equipment, including defibrillators, stethoscopes and oxygen, and former NHS ambulances, for a hospital at Idlib, in north-west Syria.

After helping to raise the funds, he is said to have insisted on joining the convoy rather than spend Christmas with his wife and two teenage children at their home in Eccles, Greater Manchester.

One of the organisers, Kasim Jameel, a taxi driver from Bolton, Greater Manchester, said "Alan is a man who is full of compassion and we are just praying to Allah that he is released safe and sound.

"We are liaising with the authorities and we do not want to say anything which might put him in any further jeopardy or which will inflame the situation. I could tell a lot of stories about the good that Alan has done and about how, as a non Muslim, he has helped Muslims who have suffered in the conflict.

"He is motivated to help others – not just by helping the convoy but by loads of other things as well.

"He is the nicest of nice guys who has done so much to help other people. He is just a normal bloke, an everyday taxi driver who wanted to do good. We are thinking about him all the time and praying that he will be allowed home to his family."

The convoy set off from Bolton on 20 December, last year, with Jameel, telling his local newspaper, the Bolton News, that they had been inspired by the death of Abbas Khan, an orthopaedic surgeon from London, who died in Syrian government custody earlier that year.

It was the second convoy organised by the same group: the previous March its members joined a group of Muslims from Scotland to take seven ambulances, medical equipment and baby milk to Idlib.

A BBC journalist who met Henning while making a documentary about aid convoys to Syria described him as a likeable and amusing man.

Catrin Nye said he told her that he had been deeply moved by his first trip to Syria. "It had been a life changing experience," she said. "He had handed out the goods. He described holding the children ... and how that really affected him. He told me he had to go back."

This year, however, British Muslims carrying aid to Syria have fallen under some suspicion, and the UK government is warning against travel to the country under any circumstances.

Jameel has been advised by police at Dover, Kent, against travelling to Syria, while the Charity Commission has warned that material carried by aid convoys may be abused for non-charitable purposes. Police have been distributing leaflets to volunteers suggesting that they should make contributions instead to established charities, and have visited some volunteers at their homes, asking them not to travel.