Former Guantánamo Bay detainee Moazzam Begg offered to intervene to help save the life of British hostage Alan Henning with an extraordinary public appeal but was twice rebuffed by the Foreign Office.

According to a revealing correspondence, the human rights activist held talks with Alistair Burt, a former Foreign Office minister, in January to secure the release of Henning, a Salford taxi driver, who was killed by Islamic State militants last week.

The paperwork indicates Begg said he had been contacted by Henning’s friends in December, just as his passport had been taken away by the home secretary for being a “terrorist risk”.

Begg – told by the Foreign Office his “help was not needed” – was arrested a few weeks later in February on suspicion of terrorism offences linked to Syria. He was held in Belmarsh prison, but was released last week after it emerged that secret intelligence material had been withheld from police and prosecutors. That had included minutes of meetings with MI5 where his travel plans were discussed.

Begg’s offer to help Henning was renewed last month from the high security prison in south London after the video that depicted the murder of US journalist Steven Sotloff also showed Isis militants threatening the life of Scottish aid worker David Haines.

Begg’s offer to make a televised appeal on behalf of British hostages was made via his lawyer, Gareth Peirce, but police did not make arrangements to interview Begg until after Haines had also been murdered. Begg’s lawyers said that apart from one phone call, “there had been no response to [his] suggested personal appeal to attempt to save the life of [Henning]”.

Begg believed that an appeal from a former Guantánamo inmate – a man who had worn the orange jumpsuit that Isis was forcing its hostages to wear – and who was at that time imprisoned as a result of his visits to Syria would have had some influence over the militants.

Such an appeal, although unusual, would not have been unprecedented: in December 2005, the radical cleric Abu Qatada made an appeal from Full Sutton prison, near York, calling for the release of peace activist Norman Kember, one of four westerners then held hostage in Iraq.

However, Begg’s offer to go public went unheeded by the Foreign Office, which instead insisted that Begg wrote out a letter. Officials told his lawyers the government “will retain editorial control of the appeal and decide how it is best used … we will issue it through private channels”.

Last night David Cameron confirmed that Begg’s help had been sought. He told BBC’s North West Tonight: “We are very happy to work with anybody. My understanding is that Moazzam Begg did make some appeals – sadly, as we know, those appeals fell on deaf ears.

“These people in Syria are absolutely brutal and really will stoop to any depths and do the most dreadful and ghastly things, and that is what’s happened. But if Moazzam Begg has information to provide about who these people are, he should provide it.”

Henning’s brother, Reg, urged Begg to make efforts to save the lives of other hostages held by Isis, including British photojournalist John Cantlie and US aid worker Abdul-Rahman Kassig. Begg, who had been in Syria working with groups fighting against President Bashar al-Assad before the rise of Isis, told Radio 4’s Today programme: “I was in Syria before Isis and before [al-Qaida’s] al-Nusra front was proscribed. I was involved in interventions when they had taken hostages. I had got other groups to pressurise them and got [people] released.”

However, he said that this time the government had been concentrating too hard on demonising him. “They simply refused to look at anything,” he said, describing the murder of Henning as despicable.

It also emerges from the correspondence that Begg had confronted senior Isis commanders in Syria and argued that their treatment of prisoners had been un-Islamic – an appeal which, his lawyers told the Foreign Office, had led to the release of a number of hostages.

Begg also drafted a personal appeal in Arabic to the leader of Isis, Abu Bakr al-Baghdadi. Begg said that because of his connections to the Arab world – he is fluent in both colloquial and classical Arabic and is well known because of his incarceration by the Americans in Guantánamo – he could have made a “connection” with the militants.

Begg said: “Alan Henning’s position in Islam … was a protected person. I have no problem [telling Isis] because that is how they see it.”

Begg said he had begun to become increasingly concerned about the rise of Isis when, during his visits to Syria in 2012: “(I had said) these people will start committing all sorts of atrocities in the name of Islam and jihad and we from our community have to start to challenge this.”

Begg said that he would advise Muslims not to go to Syria now. However, he differed from the British government in advocating talks with Islamic State. “I have advocated for negotiations with the Taliban, al-Qaida and negotiations with Isis. The Turks have been [talking] with them. They got their people out... What does Britain do? Britain says let’s abandon our people, let them die [so we can] look tough.”

Begg has said it is inevitable that he will be bringing civil proceedings against MI5 and the government.