British Muslim scholars have made a direct video appeal to Islamist militants to release Alan Henning,whom they are threatening to behead, warning the killing would break Islamic laws.

The video appeals come from three scholars from the orthodox Salafi school of Islamic thought, which many in the west see as fundamentalist.

Their plea on behalf of Henning comes as friends and colleagues of a second British hostage, the photojournalist John Cantlie, spoke of their dismay at the propaganda video he was forced to make, which was distributed through social media on Thursday.

After watching the video, in which Cantlie exhorted members of the public to call on the UK and US governments to negotiate with his captors, they spoke of a man who is resourceful and charming, but whose bravery would frequently border on the reckless.

The video in which Cantlie appears marks an unexpected departure for Isis. The group's previous three videos have depicted hostages being murdered.

Cantlie, 43, speaks directly to the camera from behind a desk, reading calmly from a script that presents him as a journalist who will be making a series of films that will "show you the truth" about Isis and western military action in the region.

One colleague, who did not wish to be named, said he suspected that Cantlie may have suggested to his captors that he should make the films: "No doubt he's doing whatever he can to save himself."

The Muslim scholars who are appealing for the release of Henning say that his detention is against the Qur'an's teachings and an injustice on a scale suffered by Muslims who have been held at Guantánamo by the US, and those held by Britain without trial.

Alan Henning. Photograph: family handout/PA

Two of the scholars in the video seen by the Guardian have been accused of sympathising with extremism, which they deny. They describe Henning as a humanitarian who travelled to Syria to help suffering Muslims. They hope their political credibility and theological learning will sway Isis.

They are hoping the videos reach Islamic State captors through social media, which the militants have proven so skilled at using themselves.

Henning, 47, who is married with two teenage children, is a taxi driver from Eccles, Greater Manchester. He fell into the hands of Isis after joining his Muslim friends on an aid convoy to Syria last Christmas. He was seized shortly after the group crossed the Turkish border.

Last weekend he appeared at the end of a video depicting the murder of Scottish aid worker David Haines, with killer threatening that Henning will be the next to die.

Imam Shakeel Begg, of the Lewisham Islamic Centre in south London, addresses the Islamic State captors as "our brothers and sisters in Islam" in his address, which quotes from the Qur'an to provide a theological justification for Henning being released unharmed.

The Lewisham Islamic Centre was allegedly frequented by Michael Adebolajo and Michael Adebowale,the killers of the British soldier Lee Rigby in Woolwich.