The prime minister last suffered the biggest ever revolt against his leadership when 122 Labour MPs voted against his policy on Iraq.

Nearly a third of Labour members defied a three line whip to join 52 Liberal Democrats and 13 dissident Conservatives in the opposition lobby.

Some 199 MPs united to tell Tony Blair that the case for war in Iraq was "as yet unproven".

The Labour rebels demanded more time for weapons inspections and insisted on the government not taking military action without the prior approval of parliament and the UN.

Former whip Graham Allen, who helped organise the rebellion, said: "This is a heartfelt plea from parliament for the prime minister to get off the escalator to war."

One senior minister told today's Guardian: "This is a really sophisticated vote. Tony now knows he has to go along the UN route, which he didn't know on his own. It needed the demonstrations and this vote and it's what Britain should do."