Diplomats in Iraq are trying to discover the fate of British hostage Norman Kember and his three colleagues.
Norman Kember has been blindfolded and shackled
A group called Swords of Truth claimed to be holding the trio, who were abducted in Baghdad last month.
They accused the men of spying and vowed to kill them unless their demands were met. But Saturday's deadline passed with no information emerging.
The British government said it was doing all it could to ensure the captives were released unharmed.
Repeated pleas for mercy were made by from senior Muslim leaders.
Mr Kember, 74, from Pinner, north London, American Tom Fox, 54, and Canadians James Loney, 41, and Harmeet Singh Sooden, 32, had travelled to Iraq as a "gesture of solidarity" with Canada-based international peace group Christian Peacemaker Teams.
The group denied the men were spies.
Friends and family of Mr Kember have endured an agonising wait for news.
Chris Cole, director of the Fellowship of Reconciliation, of which the pensioner is a trustee, said: "We are looking for a miracle at this stage."
Bruce Kent, a friend of Mr Kember for 15 years, said: "I haven't given up hope at all.
"They may be working on some way of getting off the hook without losing face.
"It's quite possible they may be preparing a video to say why they are releasing them."
Mr Kent said he would be attending a vigil for Mr Kember on Monday night from 6pm outside St Martin's in the Fields Church, Trafalgar Square, London.
Many Christians and Muslims are expected to attend.
British Defence Secretary John Reid said the government had no new information about Mr Kember.
"We are doing, through the Foreign Office, through (Foreign Secretary) Jack Straw, everything possible to try and make sure his life is saved and that of his colleagues is protected," he said.
Muslim leaders in Britain and abroad - as well as terror suspects held in the UK - have repeatedly called for the release of Mr Kember, who was seen on video shackled and blindfolded.
They also pleaded for the other three hostages to be freed.
Anas Altikriti, the former president of the Muslim Association of Britain, has been involved in negotiations in Iraq, without a breakthrough.
The BBC's Caroline Hawley in Baghdad said it appeared it had not been possible to establish direct contact with the kidnappers, despite an Iraqi government effort to gather intelligence on them.
BBC World Affairs Editor John Simpson, who is also in Baghdad, said it can take days before it is known what has happened to hostages after deadlines have passed.
Referring to the murders of previous UK hostages in Iraq - which include Ken Bigley and Margaret Hassan - he said: "It's difficult to be too hopeful about the future but, of course, there's always the possibility that this time will be different."