The Conservative leader's current line is that the party will oppose the single currency at the next election - effectively ruling out membership for another nine years under the Tories.
The party has argued that the unknown economic consequences of membership need to be tested for some years before Britain joins up to the euro, while Labour is saying that it should be possible to form an economic judgement soon after the next election.
But the latest official Tory shift against the currency is being based on the argument that it would lead directly to a political superstate, with a massive loss of British political sovereignty - effectively putting a complete Tory block on membership.
In a speech on the constitution last week, Mr Hague said in a little- noticed passage that he would be making a further speech on the "far-reaching constitutional implications of Britain's membership of the European Union" in May.
However, he also said: "We would well find ourselves some way down the road to a United States of Europe, with all that entails for national sovereignty and accountability."
That view has been strongly endorsed over recent weeks by some of Mr Hague's senior Shadow Cabinet colleagues.
Peter Lilley, the shadow Chancellor, said in an official Conservative Party press release last Friday that the Government "have signed up to EMU [economic and monetary union, the single currency] as a political project to transfer more control over our affairs to Europe."
Michael Howard, the Tory spokesman on foreign affairs, said in another party press release last Monday that "constitutional questions" about full political union were "at the heart of this decision. They cannot be ignored".
But the strongest line has been coming from John Redwood, the party's trade and industry spokesman, who recently had a press release repudiated by Mr Hague because it was offensive to the German Chancellor, Helmut Kohl.
In another press release last Monday - again issued by Conservative Central Office, with leadership approval - Mr Redwood said that a single currency would be like taking out a joint bank account with the neighbours.
"A single budget leads to a single government," he said. "Once you have entered into your joint bank account with your neighbours, they control all of your finances, how much holiday time you can have, whether you can work overtime and how much you pay for food, goods and services. You have little control left over how you run your life."
In the end, Mr Redwood concluded, it would even become "pointless" to hold general elections. "No matter who you elected, they would still be only one voice among fifteen more in Brussels and your views would not matter very much.
"Your Members of Parliament would have no more power to influence your life than an opposition councillor on council with a built-in majority," he said.
The bank account image was pursued yesterday by Mr Lilley, who told Alastair Stewart on GMTV's Sunday Programme: "We don't want to merge our bank accounts with our partners."Reuse content