Tony Blair says everything possible is being done to secure the release of a UK civil engineer kidnapped in Iraq.
Kenneth Bigley's family are 'devastated'
Militants holding Kenneth Bigley and two Americans threatened to kill them unless women held in Iraqi jails were released, in a video shown on Saturday.
"We are monitoring the situation, we are doing everything we can," said Mr Blair after the ultimatum was broadcast.
The Foreign Office has advised against all but essential travel to Iraq, warning Britons risk being kidnapped.
Gunmen seized Mr Bigley with Americans Jack Hensley and Eugene Armstrong on Thursday, from a house in Baghdad's Mansour neighbourhood.
In a video aired by Arab TV channel al-Jazeera on Saturday morning, the abductors threatened to kill the hostages within 48 hours if the US did not release all women currently detained in the Abu Ghraib and Umm Qasr jails.
Mr Blair, who is due to meet interim Iraqi Prime Minister Iyad Allawi for talks in Downing Street on Sunday, said every effort was being made to secure Mr Bigley's release.
He added he did not want to say more for the sake of the hostage.
Mr Bigley, originally from the Liverpool area, is understood to be 62 and is married with one child.
The video shows the three hostages, who are sat down and blindfolded, identify themselves and describe their jobs.
Behind them, a masked militant reads out the kidnappers' ultimatum.
He claims responsibility for the abduction on behalf of the Tawhid and Jihad (Unification and Holy War) Group, led by al-Qaeda militant Abu Musab al-Zarqawi.
His group is believed to have carried out several attacks and kidnappings in recent months, including that of American contractor Nick Berg who was beheaded on video.
According to reports, Mr Bigley had told neighbours in Baghdad he was planning to retire to Thailand with his Thai wife.
His family said in a statement: "We were devastated to find Ken had been taken, and are still struggling to come to terms with what is happening.
Foreign Office Iraq travel advice
All but essential travel to Iraq should be avoided
Essential travel should be delayed if possible
British nationals at risk in Baghdad's Mansur district
Seek professional advice on current security arrangements
"It's hard to understand why Ken would be targeted... but we would appeal to those who have taken him to please return him safely to us."
The kidnapped men, who worked for Gulf Supplies and Commercial Services - a Middle
East-based general services and construction contractor - lived together in the walled-off two-storey home.
The Briton's family added: "Our thoughts and prayers are with the families of his two colleagues too, who must share our distress."
More than 100 foreigners have been abducted by insurgents in Iraq over the past 17 months, in a bid to destabilise the interim government and drive foreign troops from the country.
The latest kidnappings brings the number currently held to 19.
It follows the abduction of two female Italian aid workers last week, and two French journalists on 20 August.
In August, UK journalist James Brandon was released by kidnappers a day after being abducted from a hotel in Basra.
British Muslim groups have condemned the kidnappings and called for the immediate release of the hostages.