Although there are still seven months to go before Scotland holds its referendum on independence, the three main political parties at Westminster have decided it is time to play hardball. George Osborne, Ed Balls and Danny Alexander have put aside their political differences and said that if Scotland votes yes, London will refuse to take part in the currency union sought by Alex Salmond.

The warning that an independent Scotland could not keep the pound is an attempt by the pro-union camp to suggest that economic policy would be left rudderless in the event that Salmond and his SNP colleagues triumph in September's poll. There are, though, a number of options available to the yes camp despite the toughening up of the rhetoric from the three main parties at Westminster.

Option one is to do nothing. Nicola Sturgeon, Scotland's deputy first minister, says the joint pledge by Osborne, Balls and Alexander amounts to bullying on the part of a campaign panicked by the narrowing of the no camp's opinion poll lead. The SNP could assume that London is bluffing, aware that the rest of the UK would suffer significant collateral damage in the event of a messy divorce that left Scotland scrabbling around for a new currency. In the short term, that is likely to be the response.

Option number two is to assume that the three main Westminster parties mean what they say and quietly agree to accept whatever terms the Treasury and the Bank of England demand. There will be a high-stakes poker game if Scotland votes yes to independence: while London does not hold all the cards, it has most of the good ones. Put simply, an independent Scotland would need a monetary union with the rest of the UK more than the rest of the UK would need a monetary union with Scotland.

Option number three would involve Scotland continuing to use the pound even if London said it couldn't. This sounds far-fetched, but the free-market Adam Smith Institute says it is both feasible and sensible. The institute's research director Sam Bowman said Panama, Ecuador and El Salvador all used the US dollar without permission, so there was no reason Scotland could not use the pound without Westminster's permission. What it would mean is that the Bank of England would not act as the lender of last resort for Scottish banks or as a guarantor for an Edinburgh government, but according to Bowman this would be no bad thing as it would deter irresponsible lending. "An independent Scotland that used the pound as its base currency without the English government's permission, with banks continuing to issue notes privately and private citizens free to choose any currency they wanted, would probably have a more stable financial system and economy than England itself".

The fourth option would be to return to the one that Salmond appeared to favour before the financial crisis of the past seven years, namely joining the euro. But emulating Ireland as the new Celtic tiger inside monetary union does not look as attractive as it did before the Irish banks went belly-up and the International Monetary Fund arrived in Dublin with an austerity programme. Moreover, negotiations with the European Central Bank and the European Union over joining the single currency would be just as difficult as those with London threaten to be, perhaps more so given that a monetary union already exists between Scotland and the rest of the UK.

Finally, there's the already spurned option of going it alone. This would involve abandoning plans for independence-lite and going for the real thing: a Scottish central bank issuing a Scottish currency and setting its own interest rates. But the new government would need to win the confidence of the markets and the means of doing so would be higher interest rates than in the rest of the UK, with tough controls over spending to boot.