The prime minister has told MPs that the alternative to military action against Iraq is a sanctions regime that results in the death of thousands of innocent Iraqis.

Speaking in the Commons yesterday, Tony Blair said that his message to anti-war protesters was that a moral case could be made for conflict.

"Before we take the decision to go to war, the morality of that should weigh heavily on our conscience because innocent people, as well as the guilty, die in a war," he said.

"The only alternative to disarmament by the UN is that we keep sanctions in place year on year on year. I'm simply saying that also is a choice with bad and devastating consequences for the Iraqi people."

Blair added that he was still hopeful that a second security council resolution authorising conflict could be achieved.

Chancellor Gordon Brown announced yesterday that he was setting aside a further £750 million to meet the costs of a possible war, in addition to the £1 billion already allocated.

The government's critics hope that this weekend's Stop the War rally in London could be attended by half a million people and dozens of Labour MPs.

Meanwhile, British hopes that chief weapons inspector Hans Blix will tomorrow deliver a tough judgement on Iraq were boosted by news that a banned missile system has been discovered.