As coalition forces press ahead into Baghdad, Tony Blair has been warned that he must secure a central role for the United Nations in a post-conflict Iraq.

Former Cabinet minister Chris Smith said the prime minister will fuel Labour tensions unless he persuades the US to back a UN-led administration to replace Saddam Hussein.

The warning came as thousands of US troops headed into Baghdad after a significant breakthrough in the war.

Smith said it was "absolutely essential" that reconstruction is decided by "the international community through the United Nations rather than just determined by the Americans".

He said the issue "is very important for the future of the United Nations and also, ironically, for the future standing of the United States around the world".

Smith said that many Labour MPs felt "bruised" by Blair's handling of the war.

"I think people respect the prime minister for the manner in which he, from deep personal conviction, decided that this was the right thing to do and went ahead with it," he told GMTV.

"I think a lot of people within the party disagree with the decision that he took."

His intervention came as ministers stressed that the war in Iraq is not over despite the recent advances.

Commons leader Dr John Reid signalled that troops will not be coming home "overnight".

"We are coming out of a situation where there is a war on. That war will not cease overnight. As long as we are coming out of a war situation into the immediate aftermath of war then the armed forces will have a role to play," he told the BBC.

Despite suggestions that the US will announce plans for the creation of an interim administration on Monday, ministers still insist the UN will play a central role in the post-conflict reconstruction of the country.

In an interview with the Sunday Telegraph, Welsh secretary Peter Hain called for the United Nations to be "put in charge" of rebuilding Iraq after Saddam is deposed.

"What is crucial is that the UN is put in charge after the interim transitional arrangements. That is vital to us and the whole of the European Union," he said.

"It is essential in order to open up the huge potential for European aid and development assistance."

The comments came as the defence secretary insisted that Saddam's regime still possesses weapons of mass destruction.

Geoff Hoon said the coalition's failure to uncover the weapons did not mean they did not exist.

He pointed to the discovery of protection suits as evidence that Saddam has stockpiles of chemical weapons.

"Everyone knows the British forces and American forces do not have such weapons, so the only reason why Iraqi forces would have been issued with this kind of protective clothing is obviously to protect themselves against their own use of chemical weapons," he said.