Three high-profile British Muslims have warned Isis (now known as the Islamic State) that killing hostage Alan Henning would be against Sharia law.
Various representatives of the British Muslim community have previously signed a letter urging Islamic State to release 47-year-old Henning.
Now, their latest plea is in the form of a video, addressed directly to Islamic State, from Shaykh Dr Haitham al-Haddad: a judge on the Sharia council in London, the chairman of Muslim Research and Development Foundation, and an imam.
Speaking to the camera, Shaykh Dr. Haitham al-Haddad says: "This is to confirm that executing this man is… impermissible, prohibited according to Sharia."
Abu Eesa Niamatullah, a director of Prophetic Guidance in Manchester, said he can "personally vouch for" Henning and that his kidnappers and would-be killers are "defacing the religion of Islam".
Imam Shakeel Begg, of Lewisham Islamic Centre in south London, quotes from the Koran in the video, explaining that there is "no justification in our religion that allows you to continue to hold him let alone harm him".
Alan Henning, a taxi driver from Manchester, was kidnapped by the terrorist organisation while working in Syria delivering aid last year.
Henning appeared at the end of an Islamic State YouTube video released earlier in September in which fellow captive, British journalist David Haines was executed.
In the video the jihadist executioner threatens that Henning will be their next victim if US and UK governments do not agree to their demands for ransoms and to end Western military presence in the Middle Eastern countries.
Footage released this week shows Henning with other aid workers on the Greece-Turkey border in December, before his abduction. He says that travelling to a dangerous and war-torn country such as Syria is "worthwhile" to help those who need it.
The use of video to deliver a message to Islamic State echoes the terrorist organisation's social media campaign, which features professional-quality videos showing the beheadings of James Foley, Steve Sotloff and David Haines; and a recent video showing English photojournalist John Cantlie reading from a script, telling viewers he will reveal "the truth" about the Islamic State.