19/12/2013 12:06 GMT | Updated 18/02/2014 05:59 GMT

The Only Way to Guarantee That We Keep the Pound Is to Stay in the UK

The credibility of Alex Salmond's currency plans for a separate Scotland suffered a further serious blow last night. One of the big three credit rating agencies, Fitch, warned that the nationalists' plan for keeping the Pound if we leave the UK would be "unstable" and lead to "high volatility and market turbulence". They make the obvious point that for currency union to work it needs political union, the very thing we have today that the nationalists want to break up.

The agency will be key to determining the credit rating of a separate Scotland if we vote to leave the UK. What it says about Alex Salmond's currency plans is important for the cost of borrowing in a new state. To dismiss their concerns, as the nationalists have done, is very foolish indeed.

Fitch now joins an increasingly long list of experts who say that a Eurozone-style currency union would not work, for either Scotland or the rest of the UK.

On one side you have lots of experts saying a currency union would be unlikely and unworkable. On the other side you have Alex Salmond saying that everybody is wrong but him. It is simply not credible. How many different experts have to give this same message to the SNP before they sit up and take notice, rather than brushing all concerns aside?

Currency is fundamental to the nationalists' economic case for breaking up the UK, insofar as one even exists. Whether it's pensions, renewable subsidies, mortgage rates or savings, Alex Salmond's plans hinge on Scotland and the rest of the UK entering into a binding agreement to veto one another's budget, tax rates and borrowing levels. If a currency union isn't possible the rest of the SNP's already threadbare economic case collapses like a house of cards.

It would appear that the idea of a currency union is now off the table. Alex Salmond can only bury his head in the sand for so long before he has to tell the people of Scotland what his Plan B on currency is. Would we join the Euro? Or would we set up our own currency?

Neither of these options is as good as what we have at the moment, but at least people in Scotland would know what they are being asked to vote for.

The currency we use is about more than just the pound in your pocket or the money you withdraw from the hole in the wall. It's about jobs in our financial services industry, the value of savings and the cost of mortgages in Scotland. These are things that affect millions of Scots directly. It is too important for Alex Salmond to gamble with.

What is clear after this latest expert intervention is that the only way to guarantee that we keep the Pound is to stay in the UK.

If you want to keep the UK Pound, join our campaign today

Alistair Darling is the Chair of Better Together and Labour MP for Edinburgh South West - this blog first appeared on the Better Together campaign blog, and can be read here