The remains of two British security guards held hostage in Iraq for two years have been identified and named.
They were highly likely to be those of Jason Swindlehurst, from Skelmersdale, Lancashire, and Jason Creswell, from Glasgow, the Foreign Office (FCO) said.
Two other guards and the IT consultant they were protecting, Peter Moore, are still thought to be captive.
Gordon Brown sent his condolences to the victims' families and called for the release of the remaining hostages.
Videos of captives
The prime minister said: "There is no justification for hostage-taking and I call on those people who are holding the other Iraqi and British hostages to release them immediately."
Consultant Mr Moore, from Lincoln, and his guards were captured by armed militants at the Ministry of Finance in Baghdad in May 2007.
They were surrounded in broad daylight by 40 gunmen wearing police uniforms and bundled into vans.
Prime Minister Gordon Brown offers his condolences to the families
The bodies of Mr Swindlehurst and Mr Creswell were handed over to Iraqi authorities on Friday night, two years after they were taken in Baghdad.
They remain in Baghdad's green zone in the care of the British Embassy. A government source confirmed that the bodies had not been mutilated or beheaded.
Forensic examiners are trying to establish the date and method of death ahead of an anticipated murder inquiry.
The kidnappers had previously claimed Mr Swindlehurst committed suicide in May 2008, but his family had said that "Jason's not that sort of guy".
In the first video released by the kidnappers, dated 18 November 2007, he was seen saying "my name is Jason Swindlehurst" with a black banner displaying Arabic writing in the background.
Mr Creswell - originally from Glasgow - was believed to have been living with his partner and two young children in Portlethen, south of Aberdeen.
BBC security correspondent Frank Gardner said: "This is incredibly tough for the families because there really was so much optimism. This was a blow out of nowhere."
Frank Gardner, BBC security correspondent
"The Foreign Office approach is a softly-softly one. Questions will be asked whether this was the right approach.
"When the BBC's Alan Johnston was kidnapped in Gaza, we made a big song and dance about it. We put the whole story on the news the whole time.
"There were operational reasons why that wasn't advisable in this case.
"The kidnappers said they wanted a media blackout, which they kept breaking, but I think there will be a lot of soul-searching about this."
The security firm which employed the dead hostages, Gardaworld, paid tribute to their bravery and said they were "deeply shocked" by the news.
A spokesman said: "These two professionals were outstanding individuals and experts who commanded the respect of all those who knew and worked with them."
The FCO said it had "grave concerns" over the safety of the three men still thought to be held. The two other security guards have been named only as Alan, from Dumbarton, and Alec, from South Wales.
A spokeswoman said it informed the families of the dead men "with deep regret".
"Our thoughts are with them at this sad time, and we ask that the media allow them privacy to deal with this news. We continue to do everything we can towards the safe release of the other hostages," she said.
Mr Moore's father Graeme, from Leicestershire, said waiting for news that he was not one of the dead men was "torture".
The 59-year-old delivery driver, from Leicestershire, said his thoughts were now with the family members of the other hostages.
"Obviously, I hope my son is alive but I feel desperate for the other families. What they are going through is unimaginable," he said.
It is possible the handover of the bodies could be seen as a trade-off for the release of militant Laith al-Khazali, on 6 June.
His freedom had been a stipulation for the hostages being freed, the BBC's Frank Gardner said.
The aim of the kidnappers, who call themselves the Islamic Shiite Resistance in Iraq, is to get their militants freed from Camp Cropper, a US military prison west of Baghdad.
Every stop was pulled out to try and find these men
Kim Howells Former Foreign Office minister
This is Britain's longest running hostage crisis since the days of Terry Waite and John McCarthy in Lebanon in the 1980s.
Little is known about the the men because of a media blackout during a large period of their captivity.
The blackout originally came on the instruction of the hostage-takers who said they did not want publicity.
The British government has a policy of not making substantive concessions to kidnappers to discourage further kidnapping.
However former Foreign Office Minister, Kim Howells, who was involved in trying to negotiate the hostages' release when they were first taken, said the government did all it could.
He told the BBC: "In the best part of four years that I was in the Foreign Office, I don't know of another operation that was better resourced, that more energy was put into.
"Every stop was pulled out to try and find these men."
The policy of not making substantive concessions has been criticised by some and praised by others, including some North African countries who have attacked other European nations for paying huge ransoms to kidnappers.
Mr Moore had been working for American management consultancy Bearingpoint when he was kidnapped, while the other men were contractors employed to guard him.
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