LONDON (AP) — Britain’s main political leaders have been scrambling over the last two days to offer Scotland greater autonomy in a last-minute bid to stave off independence, as the pound sank on opinion polls suggesting separatists are gaining ground – and may even be in the lead.

A Yes vote in next week’s referendum would end Scotland’s 307-year-old union with England and plunge Britain into uncharted constitutional and economic waters.

Many British politicians and business leaders have long considered that outcome unlikely, and for months polls have put the anti-independence side ahead. But the gap has narrowed ahead of the September 18 vote and one survey released Sunday by pollster YouGov showed the two sides nearly even.

YouGov’s methodology — which samples from a pool of online respondents who have signed up for the panel and are paid to participate — is not trusted by all poll-watchers, and other surveys gave the advantage to the No side. But the sense that the race is too close to call rattled independence opponents, and energised supporters.

“I don’t think there’s any doubt at all the momentum is with the Yes campaign,” said Scotland’s pro-independence Deputy First Minister Nicola Sturgeon.

“Increasing numbers of people are concluding the best way to protect the health service, create jobs and ensure we never get Tory governments we don’t vote for is to vote Yes.”

The leader of the anti-independence “Better Together” campaign denied his forces were panicking.

“I’m very confident we will win the day,” said Alistair Darling, a former British Treasury chief.

But the sudden move to offer greater powers to Scotland suggests the No campaign is far from confident.

Scotland joined England and Wales to make Great Britain in 1707. Now with a population of 5.3 million -one-tenth of England’s – Scotland already has considerable autonomy, with its own parliament and separate legal and educational systems.

Britain’s Conservative, Labour and Liberal Democrat parties – all of whom oppose independence – are preparing to offer Scotland even more financial independence, including greater tax-raising powers, in a bid to avert a Yes vote.

Many Scots favour that option – known as maximum devolution, or “devo max” – but it is not on the referendum ballot, which asks whether Scotland should become an independent country.

Supporters of independence accuse the No side of running a negative campaign that overstates the risks of independence.

The “Yes Scotland” campaign argues that an independent Scotland would be a prosperous country whose oil revenue would underpin a cosy social safety net. The approach has won over many Scots opposed to the budget-slashing policies of Conservative Prime Minister David Cameron’s London government.

Polls suggest rising support for independence among Labour voters and from women, who have generally been more sceptical than men of the Yes side.

The latest poll results jolted financial markets, which for months seemed to take a sanguine view of the referendum. At one stage, the pound had fallen 1.4 per cent to US$1.6104, its lowest level since November.

Shares in Scottish-based financial institutions such as the Royal Bank of Scotland, Lloyds Banking Group and Standard Life also sagged, falling by between one and two per cent.