The Campaign Group of Labour MPs issued a statement calling on the Prime Minister to "consider his position" and fellow left-wingers urged a party conference to discuss a leadership challenge.
Hilton Dawson, the MP for Lancaster and Wyre, also suggested in a Commons debate that Mr Blair should step down if he failed to get a fresh UN mandate for war. And union leaders used a Downing Street meeting with Mr Blair to warn of the dangers of going to war without a second resolution.
John Reid, the party chairman, was withering about the chances of a special party conference, pointing out that the "usual suspects" were behind the idea and it would not get beyond the ruling National Executive Committee.
Government sources also stressed that Gordon Brown, the most likely successor to Mr Blair, would probably adopt a similar policy on Iraq.