The former EU commissioner for monetary union said it would not be possible for an independent Scotland to join the EU if it used the pound without a formal currency deal or its own central bank.

Olli Rehn, who stood down in July as the European commissioner for monetary union and the euro, wrote to Danny Alexander, the chief secretary of the Treasury, this week to say having a central bank was an essential requirement of EU membership.

Now an MEP, Rehn said in a letter published on Tuesday night that Alex Salmond's apparent plan to use the pound without the formal permission of London – a policy known as "sterlingisation" – would "simply not be possible".

The intervention came as the pound fell to a five-month low against the US dollar and also weakened against the euro, hours after a YouGov poll showed for the first time that the yes campaign needed only a three-point swing to win the independence referendum.

With the vote due in just over two weeks, it showed support for independence at 47%.

Rehn said the currency issue arose because "that would obviously imply a situation where the candidate country concerned would not have a monetary authority of its own and thus no necessary instruments of the EMU" [economic and monetary union].

He wrote that a core part of any new membership negotiation was for the applicant to commit themselves to EMU, and that implied the "will and expected capacity" to meet the tests for joining the euro – a policy Salmond has repeatedly rejected.

Senior figures in the no campaign conceded the poll had accurately picked up a surge in support for independence. They denied they were rattled by the result, which showed an eight-point fall in the no vote in four weeks, but admitted they were now urgently studying its findings and reviewing their strategy.

"There's a fight to be had," said one senior figure in the pro-UK group Better Together, as sources in Labour and Downing Street predicted their attacks on the yes campaign's economic and spending plans would now be stepped up.

"We would be fools to say something hasn't shifted. We don't think it's as big as YouGov suggests but the challenge for us [is to] look at the poll and what it is telling us."

Salmond, speaking as he launched a plan to boost Scotland's food and drinks exports by £4.5bn after independence at a whisky distillery in St Andrews, said: "I have always thought we would win.

"The polls are obviously very encouraging. But I am much more encouraged by the reaction on the streets. Dundee yesterday was extraordinary – it was a carnival atmosphere with certainly hundreds of people in the city centre.

The hit on the pound, which fell in value after the poll by 0.7% against the dollar to $1.6492 – its lowest rate since March and 0.6% against the euro, is now likely to feature in the no campaign's responses.

Currency brokers warned that the pound was likely to come under further pressure as the referendum on 18 September drew nearer, leaving the Treasury braced for further falls.

Analysts believed there was only a small chance of a yes victory but the poll upset currency markets, raising the cost of betting against sharp swings in the pound's level on the markets. Investors began to insure themselves against the impact of a yes vote and one measure of hedging costs notched up its biggest one-day rise for three years. Kit Juckes, a currency analyst at Société Générale, said a yes vote would raise a range of issues, but lead to significant impacts on sterling.

"A Scottish yes to independence poses far more questions – about the currency, the debt, the oil, the future – than it answers but my best guess is that a yes would trigger a 3-5% fall by sterling as an initial reaction," he said.

The recent strength of the pound has dented British exports but ministers will be anxious about any currency movements that show a lack of confidence in the future of the currency.

Alexander said Rehn's views were a fatal blow to repeated suggestions by Salmond that an independent Scotland could use the pound freely if the UK government refused to set up a formal sterling zone.

Alexander said sterlingisation "is not only a bonkers idea, which flies in the face of any reasonable notion of what independence means and which would impose costs and risks on people and businesses in Scotland, it is also incompatible with Scotland's smooth re-entry into the EU".

The spike in yes support in the YouGov poll published by the Times and the Sun came only a few days after Salmond scored a convincing victory in his final televised debate against Better Together campaign leader Alistair Darling on BBC last week.

With yes campaigners buoyed by his assertive performance, YouGov found that 30% of Labour voters were now planning to vote yes, after a week of successful attacks on the UK government's record on the NHS launched by the Scottish government and the official pro-independence campaign Yes Scotland.

No campaigners said they thought up to 25% of Labour voters were now backing independence as opinions on both sides hardened; they admitted that put Labour, which commanded more than 1m votes in the 2010 general election, under intense pressure to step up its own party campaign.

One Labour source said the party would now focus far more heavily on Salmond's threat to renege on Scotland's share of UK debts if the sterling zone proposal was vetoed, and the threat of a £6bn spending deficit in 2016 identified by the Institute for Fiscal Studies. The yes campaign "has managed to just sweep that aside," he said.

Meanwhile it emerged that Lloyds Banking Group has finalised its contigency plans to move its registered office from Edinburgh to London in the event of a yes vote, Reuters has reported, with its Bank of Scotland subsidiary operating from Edinburgh as a foreign division of the business.

Lloyds, which is still 40% owned by the UK government and has its operational headquarters in London, had previously warned that independence would impacts on its costs, its regulatory affairs and its tax payments.

Vince Cable, the UK business secretary, has previously warned that Lloyds would face a decision to move its registered office - its legal head office to the City, if Scotland and the UK had separate regulatory systems.

This is largely because so many of its customers are in the rest of the UK, but if Scotland failed to persuade the UK to agree a formal sterling zone and had to use the pound informally, large sections of the Scottish financial sector are expected to move out of Scotland.

Lloyds has now told Reuters news agency that "the scale of potential change is currently unclear, but we have contingency plans in place. "In the event of a 'yes' vote in the referendum, there would be no immediate changes or issues which could affect our business or our customers.

"There will be a period between the referendum and the implementation of separation, should a 'yes' vote be successful, that we believe is sufficient to take any actions that we believe necessary," Lloyds said.