Mr Hague has threatened to expel any Tories who voted for the breakaway Pro Euro Conservative Party, and the row has served as an unwelcome distraction from the Tory leader's attempts to exploit Labour's reluctance to discuss the single currency during the campaign for tomorrow's European Parliament elections.
Lord Gilmour of Craigmillar, a former cabinet minister, said he would vote for the Pro Euro Conservative Party. Four former Tory MPs and four Euro MPs - Sir Julian Critchley, Sir Nicholas Scott, Sir David Knox Sir Robert Hicks, Margaret Daly, Adam Fergusson, Madron Seligman and Anthony Simpson - hinted that they might join him, expressing their "grave concern" at the hardening of the party's policy during the campaign.
"We would have wished that William Hague's party had put forward a manifesto more like that of the Pro Euro Conservative Party," they said. "Like many Conservatives, we shall find it very difficult to know how best to cast our vote on 10 June."
Tory officials dismissed the attack as "deeply unimportant" and said Sir Julian had not voted Tory in 1997.
Mr Hague denied Labour claims that he had kept the prominent pro-Europeans Kenneth Clarke and Michael Heseltine "locked away like mad aunties in the attic", and added: "I have not made any compacts or agreements with anybody."
Mr Hague insisted his policy of ruling out single currency membership in this Parliament and the next was in tune with public opinion.
His claim was reinforced by an ICM poll yesterday, showing the number of people who would vote "no" in a referendum on adopting the single currrency had risen from 53 per cent to 61 in the past month.
Mr Hague's campaign was boosted last night by a pledge of support from the former cabinet minister Michael Portillo, who appeared alongside him at a rally in Walsall, West Midlands.
Mr Portillo praised Mr Hague as "the man who has brought conviction back to British politics" and accused Tony Blair of "denying the true nature of the single currency project". He said Mr Hague had given people a choice on the biggest issue facing them.
Gordon Brown, the Chancellor, warned yesterday that the Tories would "live to regret" their hardline stance during the campaign. He played down the 12 per cent fall in the euro's value since its January launch, saying: "We have got to take a long-term view and not a day-to-day view."
The Chancellor denied that Labour was seeking to avoid discussion of the single currency. "We will continue to make a positive case," he said.
t In his last speech before standing down as Liberal Democrat leader, Paddy Ashdown attacked the "timidity" of Labour. He told a rally last night: "Labour is too scared of right-wing media moguls to stand up for Britain's real interests. It is sitting on the fence on the single currency when it should be leading the debate."