First minister says any plan other than sterling union would be second best and offers national debt-Bank of England deal

Alex Salmond has refused to be drawn on his back-up plan if an independent Scotland is blocked from sharing the pound and the Bank of England now the Conservatives, Labour and the Liberal Democrats have all ruled out a currency union.

The first minister is facing increasing political pressure to come up with a Plan B after the Labour leader, Ed Miliband, became the latest senior Westminster politician to say he would veto a deal on sterling if Scotland voted for independence next month.

But in an open letter to voters, published in Saturday's edition of The Sun, Salmond wrote: "Plan B implies settling for what's second best. And neither myself, my colleagues in the SNP, or the wider yes campaign will ever settle for second best for Scotland."

He said an independent Scotland would be "willing and able" to take on its share of national debt, but only if it got a fair share of the Bank of England.

The Conservatives, Liberal Democrats and Labour have all rejected the idea of a currency union, which would allow Scotland to use sterling and retain the Bank of England as its lender of last resort.

Miliband told Scottish business leaders in Glasgow on Friday that he would make a veto on a currency union a general election pledge if Scotland voted for independence in next month's referendum.

The Labour leader said he saw no case for recommending a euro-style currency union and accused Salmond of wrecking his own case by threatening to default on Scotland's share of the UK's debt, likely to total £1.6tn by 2016.

Asked if that sterling zone veto would be written into Labour's election manifesto if Scotland voted for independence, he said: "Yes. I'm ruling it out now, yes."

More than two thirds of voters surveyed for a poll in the Daily Mail said they believed the Scottish government should draw up an alternative to a currency union before the referendum, while 18% said it did not have to do so and 13% were undecided.

The poll of 1,010 people taken on 6 and 7 August – two days after a televised debate between Salmond and the Better Together leader, Alistair Darling – found that half believed Scotland should remain in the UK, while 37% intended to vote for independence and 19% were undecided. Some 53% of those polled believed the former chancellor won the debate, with 28% backing Salmond and 19% undecided. The debate was not going to change the way almost two-thirds intended to vote, while 22% were now more likely to vote no and 13% were more likely to vote yes, the poll found.

In February, the chancellor, George Osborne, and the Treasury decided to veto a deal on sterling, arguing that it would require UK taxpayers and the Bank of England to underwrite Scottish banks and Scottish government budgets.

On Thursday, Salmond reiterated a threat to renounce Scotland's share of the UK debt if Westminster vetoed a deal to share sterling.

Miliband said the first minister's warnings on debt default would not help the case for independence after Salmond's faltering performance in Tuesday's debate.

"If alarm bells were ringing after Tuesday night [about Salmond's currency proposals] when there were no answers, they should be ringing much, much louder if they're now threatening to renege on their debts," he said.

John Swinney, Scotland's finance secretary, said on Friday that rejecting a sterling union would greatly increase business costs for UK companies operating in Scotland, and leave the Treasury paying off Scotland's share of UK debt.

"They cannot turn round to the electorate in the rest of the UK and say we are going to let the Scots go away debt-free from the United Kingdom, we are going to let them off from an annual cost of £5bn," he told BBC Radio Scotland. "That's why they will agree to a currency union after independence."