Tony Blair has urged former members of Saddam Hussein's Baath Party to start working for the reconstruction of Iraq.
Mr Blair was appealing directly to Sunni Muslims
He said it was "extremely important" to give a strong message to Sunni Muslims and even former Saddam supporters that they would not be excluded.
In an interview with the BBC Arabic Service, Mr Blair said Saddam should be tried in Iraq if the Iraqis have a "proper independent judicial process".
Mr Blair said he had no doubt that Saddam had weapons of mass destruction.
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In a separate interview, the UK prime minister gave thanks to British troops across the world for the "fantastic job" they did in winning the war on Iraq and also for keeping the peace in that region.
He reiterated the Iraq Survey Group's October report which found "massive evidence" of a "huge" system of secret laboratories in Iraq.
But he was keen to offer reassurance to those, like Saddam Hussein, who are Sunni Muslims, that they will not be left out of efforts to reconstruct the country.
In his appeal through the BBC Arabic Service, Mr Blair said: "What is extremely important is for us to give a very strong message to the Sunni population in Iraq that they have a place in this new Iraq."
He said they were "not going to be excluded" and he wanted the country to be "truly representative of all the people in Iraq".
"And it is even true that there are people in the ... who were in the Baath Party under Saddam, who were there, not because they wanted to be, but because they had to be, and we should also be reaching out to some of them also," Mr Blair said.
'Tried at home'
British ministers hope that the capture of Saddam Hussein will remove a figurehead for those opposed to the US-led coalition and also remove an element of fear which may have prevented some from co-operating with the rebuilding of Iraq.
He said the future trial of Saddam "should be left to the Iraqis to decide provided they have a proper and fair and independent judicial system".
In a separate interview, he told British Forces Broadcasting Service: "We only try people internationally if there isn't the capacity within a country of trying people for the crimes they have committed against the citizens of that country.
"There is something like the remains of 400,000 human beings that have been found in mass graves already.
"There is a very strong feeling in Iraq that he should be tried in Iraq."
Mr Blair said the capture of Saddam had been "fantastic news" and "a great day" for Iraqi people who finally realised they had been liberated from Saddam.
'Developed for a purpose'
He said terrorists, some foreign and some Saddam sympathisers, would continue their attacks in the short term.
When asked if he hoped the capture of Saddam would lead coalition forces to Iraq's weapons of mass destruction, Mr Blair said: "There is obviously that possibility there, but I think in any event, we have got to carry on the work that we are doing."
He then stressed: "The Iraq Survey Group has already found massive evidence of a huge system of clandestine laboratories, workings by scientists, plans to develop long range ballistic missiles.
"Now frankly, these things weren't being developed unless they were being developed for a purpose ...
"I just say to people continually, when a country with a ruler like Saddam tries to hide what it's doing in a large country like Iraq, it is relatively easy to hide it."
Later Downing Street said the prime minister's remarks referred to the interim report published by
the Iraq Survey Group earlier this year and not to any new findings.
Asked what his Christmas message to British forces was, Mr Blair replied:
"This year, more than any other, to give thanks to them, not just on my behalf,
but on behalf of the whole country.
"Whatever people thought of the decision to go to war in Iraq, everybody
supports our troops themselves - they have done a fantastic job."