British hostage Kenneth Bigley has been shown in a new video screened on the Arab television station Al-Jazeera.
Ken Bigley was shown sobbing behind bars
The engineer is seen shackled in a cage wearing an orange jumpsuit and pleading for help from the Prime Minister.
"Tony Blair is lying. He doesn't care about me," Mr Bigley sobbed, looking unwell and distraught.
Mr Blair said "everything possible" was being done to secure Mr Bigley's release but said the hostage takers had not made contact.
"We can't make contact with them and they have made no attempt to make contact with us.
"If they made contact with us, it is something we would immediately respond to," the Prime Minister said.
Mr Blair, who was speaking ahead of a fringe meeting at the Labour Party conference, said he was "absolutely sickened" by what had gone on.
In the footage Mr Bigley calls on Mr Blair to meet the demands of his kidnappers to free female prisoners in Iraq and says his captors do not want to kill him.
Following the broadcast of the video, the Bigley family renewed their appeal for the release of the 62-year-old engineer.
"We would like to send a message, on behalf of our family, to the people
holding Ken... We want to thank you for this opportunity to see him alive again," said Mr Bigley's brother Philip.
"Please could you pass on once more our love and thoughts to him."
The family also emphasised the strain which the ordeal was placing on Mr
Bigley's mother, who is in hospital being treated for stress.
"His continued captivity is putting great strain on his mother, Lil, my grandmother," said Craig Bigley, Kenneth's son.
"From her hospital bed, she has asked us to pass on her love to her son."
The two Americans taken hostage with her son 13 days ago in Baghdad have been beheaded.
Leader of the House of Commons Peter Hain told BBC News Interactive he could "not imagine the horror" that Ken Bigley and his family were experiencing.
However, he said "it was not possible to accede to the demands of hostage-takers because that will encourage them to keep doing it to other British citizens".
He described the hostage-takers as "evil," adding "we have not seen anything like this, arguably, since the Nazis, if ever - this practice of beheading in public".
16 Sept: Two Americans and Ken Bigley taken hostage
18 Sept: All three appear in video with captors
20 Sept: American Eugene Armstrong beheaded
21 Sept: American Jack Hensley beheaded
22 Sept: Ken Bigley appears on video, appealing for help from Tony Blair
29 Sept: Second video plea made by Bigley
Ahmed Al-Sheikh, editor-in-chief at Al-Jazeera, said the network decided not to show much of the footage, not only because it is unpleasant, but also because of its political impact.
Al-Jazeera said the station had received the tape anonymously.
The Foreign Office has not yet commented on the Al-Jazeera video, but said it was checking several messages and postings on the internet claiming Kenneth Bigley was about to be released.
Mr Bigley was taken hostage by the hardline Tawhid and Jihad group along with the two American colleagues.
He has appeared in videos before, including on 22 September when he pleaded for his life.
Liberal Democrat Party president Simon Hughes told BBC News 24: "If it is a recent video it means Mr Bigley is alive and that must be a reassurance."
He said all parties backed the government's view that you don't "give in" to terrorists.
Mr Bigley reportedly urged Iraqi women prisoners be released
But he added: "We hope that the authorities in Iraq, on the issues of the women prisoners, will be able to make their own independent decision without pressure from America or anybody else - if they need to be detained, they should be detained. If they should be released, they should be released."
News of the tape was also welcomed by the chairman of the Liverpool Mosque and Islamic Institute.
Mohammad Akbar Ali said: "This would seem to offer some further hope that he will be released alive. We look forward to his release with great delight."