LONDON  - Supporters of the United Kingdom began a fightback on Monday to stop Scotland voting for independence in next week’s referendum after an opinion poll put the separatists ahead for the first time.

The shock survey put the “Yes” campaign two points ahead after months of a strong lead by unionists, causing the pound to slump to a 10-month low on fears that a break-up of the 300-year-old union was now a real possibility. The leader of the Better Together campaign, Alistair Darling, insisted that other polls put the unionist campaign ahead but admitted it was “clearly very tight” ahead of the September 18 vote.

“We’re in the position now where every voter in Scotland could potentially tip the balance in this referendum. But I am confident that we will win,” the former finance minister told BBC radio.

The unionists are planning this week to publish a “plan of action” to give more powers to Scotland, detailing a timetable and process to give further tax and spending powers to the devolved government in Edinburgh.

Senior politicians from the opposition Labour party, including former prime minister Gordon Brown, will also hit the campaign trail on Monday amid signs that growing support among their voters for independence is driving the narrowing of the polls. Scottish First Minister Alex Salmond, the leader of the pro-independence Scottish Nationalist Party (SNP), warned of “panic” in the unionist camp.

His deputy, Nicola Sturgeon, told the BBC on Monday that the pro-independence campaign was “still... the underdog in the referendum, but there’s no doubt the momentum is towards ‘Yes’.

We will continue to work flat out over the remaining ten days of the campaign.”

This weekend’s poll has shaken up a campaign that until just a few weeks ago looked almost certain to end in defeat for the independence campaign.

The YouGov poll in The Sunday Times newspaper gave the “Yes” camp 51 percent support compared to the “No” camp’s 49 percent, excluding undecided voters. Six percent said they had not made up their minds.

The two-point gap is still within the margin of error but Peter Kellner, the president of the YouGov pollsters which carried out the survey, said it was a major development.

“The ‘Yes’ campaign has not just invaded ‘No’ territory; it has launched a blitzkrieg,” he wrote in a blog posting.

The poll finding was front-page news on British newspapers on Monday, with many running the same headline: “Ten days to save the union.”

Amid continued uncertainty about the effect that independence would have on the British economy, including whether an independent Scotland would be able to use the pound, the currency fell to under $1.62 on Monday, its weakest since November. Against the euro, it fell to 80.26 pence, a three-week low.

Scotland-based financial institutions also suffered in early trading, with Royal Bank of Scotland, Lloyds Banking Group and Standard Life shares falling by more than two percent.

The poll has increased the pressure on British Prime Minister David Cameron, who agreed to a referendum but has been accused of failing to fight hard enough to keep Scotland in the UK.

Media reports suggest that some lawmakers in his Conservative party are discussing whether to call a vote of no confidence in the premier in the event of a “Yes” vote.

However, Cameron has insisted he has no intention of resigning and will lead the Tories into the next British general election in May next year.

Cameron spent the weekend with Queen Elizabeth II at her Scottish summer retreat in Balmoral, where the referendum is likely to have been a topic of discussion.

Officially the monarch has remained neutral, although some newspapers quoted royal sources as saying she is “horrified” at the prospect of a break-up of the UK.

London Mayor Boris Johnson, a Conservative, warned in a newspaper article on Monday that independence would be “an utter catastrophe for this country”.