He told a Talk Radio phone-in that the United Kingdom would be damaged by an ill-founded Euro-currency, and if he was still Prime Minister, he would do his utmost to stop it when membership selection took place in a year's time.
But the problem any prime minister would face is that no one country has a veto over single currency membership. Selection is by qualified majority vote, and the UK would need to team up with other countries to block an individual applicant.
Even an alliance between the UK, Denmark, Sweden, Finland and Greece would not add up to enough votes to create a blocking minority. But Mr Major said a single currency "botch" would create unemployment in the UK.
"The first thing that would happen if they made a botch of it, and had a weak, insubstantial European currency, is that all the money that flows into Europe would then go into the hard currencies, the strong currencies - the Swiss franc and sterling.
"That would push up the exchange rate of sterling, and that would make all our exporters uncompetitive.
"We would have done nothing wrong, but suddenly the way the world's markets operate, they buy sterling, they put up our exchange rate, we'd be uncompetitive, and we'd lose jobs at home."
Mr Major said he would make every effort to stop countries that were not economically fit from joining up to the single currency. "If they were likely to go ahead in the wrong circumstances, it is in the British national interest to stop them doing so," he said. "Everyone who goes into the single currency would need the approval of their European partners in order to do so.
"I would be prepared to vote against countries going in that were not in the right economic circumstances."