The deadline set by kidnappers holding British peace activist Norman Kember and three colleagues has passed with no news of their fate emerging from Iraq.
Norman Kember's kidnappers have claimed he was a spy
A group calling itself Swords of Truth claims to be holding the men, who were abducted in Baghdad last month.
Accusing them of being spies, it said each would be killed if all prisoners in Iraq were not released by Saturday.
Home Office minister Hazel Blears said "everything that can be done is being done" to try to secure their release.
Mr Kember, 74, of 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 (CPT).
The group denies the men are spies.
Mrs Blears told the BBC: "I think every single person obviously is desperately hoping that Norman Kember is going to be released together with the other hostages. This is a desperate situation for people to find themselves in."
BBC World Affairs Editor John Simpson, in Baghdad, said: "The deadline has passed and we are now entering new territory.
"It takes such a long time before we hear what has happened - the deadline passes and sometimes two to three days can pass before we know what has happened."
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."
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.
"But we have no further indication of any movement as of this morning," he told Sky's Sunday with Adam Boulton.
Muslim leaders in the UK and abroad 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.
Several Sunni clerics called for the release of the four men during Friday prayers at mosques across Iraq.
And last week a plea by terror suspect Abu Qatada, from Full Sutton jail, near York in northern England, was broadcast in the Middle East.
The Reverend Alan Betteridge, a friend of the British hostage, said: "My worry is, how do the captors extricate themselves from this without losing face?
"Either they lose face if they free them, or they are really out on a limb if they execute them.
"Somebody has to find a way to let them out of this - if they could find another group to whom they could release them."
Mr Betteridge, president of the Baptist Peace Fellowship, said it was a "concerted voice from the Muslim world" calling for the hostages' release.
""That is unprecedented. Whatever the outcome, something good is happening.
"The future can build on that sort of understanding, that this is the way that the West can talk to the other parts of the world."
Questioned about the worrying security situation in the country, Dr Reid said it must "not deter Iraqis coming out and showing their support for the new freedoms and democracy they have".
"That is why this Thursday's elections, the first big general elections in Iraq, are so important."