It can either start small, with a few countries that have firmly established the convergent economic stability that is required, or it can fudge the terms of entry and allow even the most profligate applicants to join up.
The Treasury preference is for a hard-core foundation which can be built on securely as other countries fall into step on convergence.
As John Major said in his On the Record interview yesterday, the alternative could be too dire to contemplate - because it could fail and the consequences would hit all members of the EU, whether they belonged to the single currency or not.
He said that if a weak country crept into the currency, its uncompetitive vulnerability would soon be exposed "and it would get very large amounts of unemployment, structural unemployment, on a very large scale.
"What would happen then?" he asked. "Then they would call everyone together, and say, `We must find some expenditure to help this country out of its difficulty.'
"Who is going to pay for that?" The Prime Minister said that none of the big contributors - Germany, France or the UK - would want to bail out the weak link in the chain.
"That is not a club we would want to belong to," a senior Government source told The Independent last week. "It would do immense damage and the Prime Minister should cast his vote against it."
Yesterday, Mr Major made it plain that he would do just that. "There's no point in it coming together for one single day if some countries are going in different directions. It needs to be a sustainable position."
There is a strong Treasury view that weak economies will not be allowed into the single currency, if only because German public opinion would not wear it.
In evidence to the House of Lords European Communities Committee Gus O'Donnell, the Treasury's deputy director of macroeconomic policy, pointed out that the Maastricht treaty referred to "the durability of your fiscal position; the sustainability". That was the message repeated by the Prime Minister yesterday. For the moment, he appears to be pushing at an open door.Reuse content