Moazzam Begg says he wanted to send a video message to those holding Alan Henning
Former Guantanamo Bay detainee Moazzam Begg offered to help secure the release of British hostage Alan Henning from Islamic State, he has told the BBC.
Mr Begg, 46, said he thought he knew who had been holding the aid worker but said the government rejected his offer.
He said Mr Henning's friends had sought his help and he had told the government he was going to intervene regardless.
The Foreign Office said it had a long-standing policy of not commenting on how it handles kidnap cases.
"The safety of British nationals is paramount," it added in a statement.
Mr Begg, who has just had terror charges against him dropped, said Mr Henning's murder was "despicable".
He told BBC Radio 4's Today programme he had helped secure the release of hostages from extremists in Syria in the past.
"I intervened by getting some other groups who could pressurise them to release those individuals and I got them released," he said.
"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."
'Exclusive to me'
IS released a video on Friday purporting to show the beheading of Mr Henning, and has 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 the for the sole purpose of securing Mr Henning's release.
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."
Alan Henning was delivering aid in Syria when he was kidnapped
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 they finally visited him on the day that Mr Henning was first shown on an Islamic State video. He said the government wanted him to deliver a message through an intermediary of their choosing.
Mr Begg walked free 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.
But Mr Begg has since claimed that Britain's security service MI5 were aware of his movements in Syria.
The Guardian newspaper has reported there were documents which include "minutes of meetings that MI5 officers and lawyers held with Begg, at which he discussed his travel plans and explained he was assisting opposition fighters in their war against Bashar al-Assad's regime".
Mr Begg was held in the US military prison at Guantanamo Bay in Cuba for nearly three years. He had been detained in Pakistan in January 2002 and taken to Bagram internment centre in Afghanistan, where he was held for about a year before being transferred to Guantanamo.
He was not charged with any offence while in US custody.
Britain is taking part in coalition air strikes against IS extremists, who control large swathes of Syria and Iraq after rapid advances through the region this year.
The latest strike by RAF jets occurred on Sunday night when RAF jets used precision-guided bombs to attack IS fighters at a fortified building near Ramadi, west of the capital Baghdad.