But pro-European Tories immediately vowed to continue their campaign for Britain to join the single currency, whatever the outcome of the ballot.
A decisive majority is expected to support Mr Hague's policy of opposing British membership in this Parliament and the next. The result will be announced on the eve of next month's Tory conference, which had looked certain to be overshadowed by the party's civil war over Europe. The leadership feared a plot to hijack the event by pro-Europeans, who said last night that the referendum "smacked of panic".
Although Mr Hague's surprise move may ensure a less troublesome week in Bournemouth, his hopes of drawing a line under the issue that has bedevilled his party were dashed when Kenneth Clarke, champion of the Tory Europhiles, made clear he would not be muzzled.
"My view of the national interest will not be changed by the result of a snap vote of party activists," said the former chancellor. "We should not join now but, if the single currency succeeds, we should stand prepared to join it as and when it is in our interest to do so."
Mr Clarke snubbed Mr Hague's offer to allow the pro-Europeans to put their case in a mailshot to party members. In an attempt to undermine the impact of the expected result, Mr Clarke said he would not campaign before the ballot - adding that any leader was bound to win a good majority in such a vote of confidence.
However, Mr Hague insisted: "As long as our party is distracted by the endless debates on the single currency, we will always have one hand tied behind our back as we fight this Government."
He said the issue was one of the largest contributors to last year's crushing general election defeat, and the party still suffered from continuous media reports of divisions. Its members were "sick and tired" of the way different internal groups had tried to "pull policy one way or the other".
Denying that he was making a plea for unity, Mr Hague declared: "I believe that unity comes through leading, not pleading. I believe it comes through consistency, through clarity, through certainty."
Last night, close allies likened Mr Hague's move to the decisive leadership shown by Margaret Thatcher as prime minister and to Tony Blair's decision to scrap Clause IV in 1994. "The crucial thing is that they were strong leaders and people knew clearly where the party stood, even though a minority opposed them," one senior Tory said. "We lost that clarity under John Major because the divisions on Europe went right up to the cabinet."
Mr Hague said all frontbenchers would have to support his policy during the referendum campaign or face the sack, but MPs would not lose the party whip if they supported the single currency in Parliament. "It is not about driving people out of the party," he said.
This freedom was seized on by Mr Clarke and other pro- Europeans, who vowed to continue to state that Britain should join the single currency in the next Parliament, if it was in the national interest.
Stephen Dorrell, a former cabinet minister, said: "Frankly, I don't think this will resolve anything because these are important issues of principle."
Ian Taylor, who resigned from the Tory front bench last autumn over the party's policy on Europe, warned: "This might appear to boost William Hague's leadership and the party will obviously endorse the leader.
"But since the single currency issue will be determined by events beyond his control, it will not boost his leadership as far as the country as a whole is concerned."
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