Cabinet ministers get on the phones to drum up support for crucial vote

Cabinet heavyweights mounted a telephone blitz of Labour MPs yesterday in an attempt to limit the scale of the rebellion when the Commons holds its vote on Iraq.

Potential rebels, who have received up to three calls from Labour whips, were surprised when they were telephoned by a senior minister pleading with them to support the Government. Those involved in the ring-round included the Chancellor, Gordon Brown, the Deputy Prime Minister, John Prescott, the Foreign Secretary, Jack Straw, and Margaret Beckett, the Environment and Rural Affairs Secretary.

Mr Blair's long-term survival as Prime Minister could hinge on the scale of the rebellion at 10pm tonight in the most crucial vote since Labour came to power in 1997.

When 121 Labour MPs voted against the Government over Iraq last month, party whips feared another 50 could join a rebellion if Mr Blair failed to win a new UN resolution. Although he has not achieved that, the Commons arithmetic has changed since the last vote. The betting at Westminster last night was the rebellion would be on a similar scale.

Some MPs who backed Mr Blair reluctantly last time but threatened to defy him in the absence of a UN mandate have changed their minds and will support the Government.

Those MPs cite several reasons. Several have been won round by the Government's attacks on Jacques Chirac, the French President, for saying last week that France would veto a pro-war resolution under any circumstances. That played into Mr Blair's hands: although he set much store by a new UN mandate, he also gave himself an escape hatch by saying that he might ignore an "unreasonable" veto.

Another factor is a backlash among mainstream Labour MPs against the attempt by the hard-left Campaign Group of MPs to use the Iraq crisis to topple Mr Blair by asking Labour's national executive committee to call a special party conference – the mechanism needed to trigger a leadership election.

"The main issue is not Tony Blair; the main issue is Saddam Hussein," said Tom Harris, MP for Glasgow Cathcart, who has been wavering since last month's debate but has decided to back the Government. Partly because of the hard left's move, which he described as "nonsense", he predicted Mr Blair would win a big majority of Labour backbenchers.

Labour whips believe that several MPs in marginal seats, who feel their owe their presence at Westminster to Mr Blair, swung back into the government fold when the Prime Minister's future seemed at risk. Others were angered by Clare Short's surprisingly personal attack on Mr Blair's "reckless" strategy on Iraq and voiced their displeasure when the Parliamentary Labour Party met last week.

How the vote will be judged

What everyone will be watching for tonight -- not least Tony Blair -- is whether the rebellion is bigger or smaller than last month's vote by 121 Labour MPs against the Government.

What the voting will mean

FEWER THAN 100 a relief for Mr Blair, who could claim the tide had turned his way since last month's vote.

MORE THAN 121 the biggest Commons rebellion by members of a governing party.

132 OR MORE a body blow to Mr Blair, as it would mean he could not command the support of more than half of Labour backbenchers.

173 OR MORE danger signals flashing for Mr Blair as, depending on the number of abstentions, he would have to rely on the votes of Tory MPs to win a Commons majority. Mr Blair could be in jeopardy if the war went badly.

206 OR MORE meltdown for the Prime Minister. More than half of Labour's MPs, including ministers, would have voted against him. He might even be forced to quit after the war.