Moazzam Begg offered help to free hostage Alan Henning
Ex-Guantanamo Bay detainee Moazzam Begg says his offer to help secure the release of hostage Alan Henning from Islamic State was rejected, months before the Briton was killed.
Mr Begg told the BBC he thought he knew of some individuals linked to those holding Mr Henning.
Prime Minister David Cameron urged him to share any information he had.
Whitehall officials confirmed the government did not accept Mr Begg's offer of help at first, but did later.
The prime minister said the government was "very happy to work with anybody" to secure the release of hostages being held by Islamic State (IS).
The Foreign Office said it had a long-standing policy of not commenting on how it handled kidnap cases.
'Deaf to all appeals'
Mr Begg said Mr Henning's murder was "despicable" and "indefensible".
He told the BBC he had helped secure the release of hostages from extremists in Syria in the past.
"The problem is that the government in its attempts to demonise and criminalise me simply refused to look at anything to do with what I was about," he said.
BBC security correspondent Frank Gardner said Whitehall officials had told him Mr Begg made two offers to appeal to IS militants.
He said the first offer - in January this year - was assessed and turned down, while the second - in September - had been accepted as the situation had "changed" by then.
But our correspondent said it was "questionable" whether Mr Begg's involvement would have made any difference as Mr Henning's captors had "ignored all pleas for mercy".
A number of British Muslim scholars, imams and community leaders called for his release, along with people holding anti-Western views - including some close to al-Qaeda.
"No-one, it seemed, had sufficient influence with his bloodthirsty captors. They were deaf to all appeals," our correspondent said.
Some Muslim groups in the UK have since criticised Mr Begg over his comments.
Hanif Qadir from the group Active Change Foundation said the claim that Mr Begg might have been able to help Mr Henning was "simply untrue and irresponsible".
Who is Moazzam Begg?
- A British citizen, he moved from Birmingham to Afghanistan in 2001 along with his family
- Moved to Pakistan in 2002, when the war in Afghanistan started
- Arrested in Islamabad in January 2002 and taken to US detention centre in Bagram, Afghanistan
- Transferred to the US-run military prison at Guantanamo Bay, Cuba, in February 2003
- Never charged with any offence and released in January 2005, when he returned to UK
- Became campaigner on behalf of terror suspects denied legal rights
- Arrested in UK in February 2014 on suspicion of seven terror charges relating to Syrian conflict, including allegation of funding terrorism overseas
- Charges dropped and freed from Belmarsh Prison on 1 October after prosecutors said they had "new material"
IS released a video on Friday purporting to show the beheading of Mr Henning; it had previously released videos showing the apparent beheadings of two US journalists, James Foley and Steven Sotloff, and British aid worker David Haines.
Mr Begg, from Birmingham, said he was approached by Mr Henning's friends in December 2013, just after the 47-year-old taxi driver from Eccles, Greater Manchester, was taken hostage.
He said he spoke to former Foreign Office minister Alistair Burt to explain that he would be making calls to IS, but it was for the sole purpose of securing Mr Henning's release.
'Exclusive to me'
Mr Begg said: "I sat with him and spoke with him for quite some time, we had exchanges, phone calls, text messages. In the end they said that they didn't need my help.
"I said that I didn't come here to seek your help, I simply came to register with you the interventions that I'm going to try to make.
"A few weeks later literally I was arrested."
Mr Begg was arrested in February on charges connected to the conflict in Syria and sent to Belmarsh Prison in south-east London.
Through his lawyer, he asked for another meeting with the Foreign Office to inform them that he wanted to send a video message to IS.
"The message that I was going to deliver could probably not have been delivered by anybody else because of the language, the terminology, the understanding, the connection that I could have made to that world was very specific and exclusive to me," he told the BBC.
He claimed the government did not respond for three weeks but officials finally visited him on the day that Mr Henning was first shown on an Islamic State (IS) video. He said the government wanted him to deliver a message through an intermediary of their choosing.
Mr Begg said IS did not exist when he was in Syria but he did encounter people from other Arab nations and Russia and Chechnya, who went on to join the group.
Conservative MP Mr Burt posted a message on Twitter confirming he had met Mr Begg "at his request", saying he had then passed information to the Foreign Office.
Mr Cameron told the BBC that if Mr Begg still had information about "who these people are, he should provide it".
Asked about the government's response to hostages, he said: "I know as prime minister that we did everything we could to try and find him and to try and get him but we weren't successful."
He said the government's approach had been to work with families and not raise the media profile to avoid putting hostages at "even greater risk".
The brother of Mr Henning, meanwhile, said he would have liked "anyone to have saved Alan's life, regardless of their history or background".
He called on Mr Begg to make the same efforts to save the lives of all hostages in Syria.
Mr Begg was freed from prison last week after seven terrorism-related charges against him were dropped. His imminent trial was abandoned after "new material" emerged.
He had been charged with attending a terrorist training camp in Syria between 9 October 2012 and 9 April 2013.
He was also accused of possessing documents for a purpose connected to terrorism and terrorist funding.