Britain still expects to secure backing for a second resolution on Iraq, Tony Blair has told the House of Commons.

During a session of prime minister's questions dominated by the issue of Iraq, the prime minister said the UN could still prove it had the will to disarm Saddam Hussein.

"I believe that we will have support for a second resolution," he said.

Quizzed by Tory leader Iain Duncan Smith, he added that it "would be unreasonable" for any member of the security council to veto the second resolution if Iraq continued to be in breach of existing UN demands.

"It certainly would be an unreasonable veto if Iraq is in material breach and we don't pass a resolution, because resolution 1441 made it absolutely clear that Iraq had a final opportunity to comply," said Blair.

Charles Kennedy said the prime minister's commitment to act through the UN meant that he must wait for the final verdict of Hans Blix.

"Would the prime minister acknowledge that unless the UN weapons inspectors were to conclude that the inspection process had failed it would be quite wrong for this country to take part in a pre-emptive military strike," he said.

Blair rejected Kennedy's argument, saying the issue was simply whether Saddam Hussein had fully complied with UN resolution 1441.

"I think the case we have set out in respect of Iraq is a good case," he said.

"I hope that if people listen to it and study it in detail they will accept that if we do have to act and go to war, it will not be because we want to, but because of the breaches by Saddam Hussein of UN resolutions."

Kennedy warned there would be wider consequences if Britain and the US decided to go to war next month.

"If the prime minister is not prepared to rule out precipative military action, then isn't the greater consequence that the international coalition against terrorism would itself be shattered?" Kennedy asked.

The prime minister repeated his warning that the greater threat was that the authority of the UN would be undermined if Saddam Hussein was not brought into line.