LONDON — Shaken by polls showing momentum shifting toward independence for Scotland, the British government will offer proposals for greater political and fiscal autonomy for the Scots if they vote to remain within the United Kingdom in a referendum on Sept. 18, George Osborne, the chancellor of the Exchequer, said on Sunday.
The narrowing polls have caused considerable anxiety among politicians and business leaders, driving down the value of the pound and raising questions among investors about the stability of the economy and the fate of the current British government.
The vote, which could bring an end to the 307-year union between Scotland and England, is also regarded as important to the future of the British prime minister, David Cameron. As leader of what is still formally known as the Conservative and Unionist Party, Mr. Cameron, already facing internal divisions over Britain’s membership in the European Union, may not survive politically if Scotland votes to break away from the United Kingdom in a referendum that he negotiated with the Scottish National Party and its leader, Alex Salmond.
On Sunday, Mr. Osborne, a close ally of Mr. Cameron’s, responded to the tightening race by promising more powers to Scotland if it votes no.
“More tax-raising powers, much greater fiscal autonomy,” Mr. Osborne told the BBC. “More control over public expenditure, more control over welfare rates and a host of other changes.”
The plan will be revealed “in the next few days” after the government gets agreement from all three major parties in the British Parliament, including the Labour Party and the Liberal Democrats, Mr. Osborne said.
“Then Scotland will have the best of both worlds,” he said. “They will both avoid the risks of separation but have more control over their own destiny, which is where I think many Scots want to be.”
That position is sometimes known as “devo max,” or maximum devolution, an alternative that Mr. Cameron did not allow Mr. Salmond to put on the ballot. Instead, Mr. Cameron insisted on a simple yes or no vote for independence. In return, he allowed Mr. Salmond, Scotland’s first minister, to extend the vote to people ages 16 and over but limit it to those who are registered in Scotland, which excludes many Scots living and working elsewhere in Britain.
Mr. Salmond dismissed Mr. Osborne’s proposals on Sunday as “a panic measure.” Mr. Salmond’s deputy, Nicola Sturgeon, calling the new polls a “very significant moment” in the campaign, said the offer of new powers had come very late.
“I don’t think people are going to take this seriously,” she told Sky News. “If the other parties had been serious about more powers, then something concrete would have been put forward before now.”
Alistair Darling, the former Labour cabinet minister who leads the “no” campaign, known as “Better Together,” said that the polls showed the referendum would “go down to the wire” but that his side would win.
“We relish this battle,” he said. “It is not the Battle of Britain. It is the battle for Scotland, for Scotland’s children and grandchildren and the generations to come.”
The anxiety in Westminster has been building, and even Queen Elizabeth II, who is also queen of Scotland, was said by The Sunday Times of London to be worried about preserving the union of the two crowns, which dates back to 1603, a century before the political union.
Mr. Salmond has said she would remain monarch of an independent Scotland, but the queen — whose public stance is one of strict neutrality — was also reported to be concerned about what independence could mean for the self-governing Church of Scotland.
Business leaders are also taking the prospect of dissolution more seriously, especially given the statement by all three main British parties that an independent Scotland would not be able to use the British pound.
Mr. Salmond has said Scotland could use the currency regardless of the result, much as Panama uses the American dollar, and has vowed to renege on Scotland’s share of British debt if it is not allowed to share in the pound.
But if the British parties follow through, Scotland will have no say in the Bank of England and in monetary policy governing the pound, undermining independence. Even Scottish banks would almost surely have to move their headquarters south, because only the Bank of England could serve as a true central bank and lender of last resort.
There are also serious questions about how Scotland would finance itself, given its dependence on royalties from the flow of North Sea oil and gas, which has been declining. A vote for independence would also mean tense negotiations with London on issues like fishing rights and the future of Britain’s nuclear submarine base in Scotland.
Questions like these are what the “no” campaign is counting on — that Scots will not want to leap into such an uncertain future, especially with the promise of maximum devolution should they stay. But many Scots see the referendum as their best and perhaps last chance to reclaim independence.
Scotland tends to vote more left-wing than England and favors a larger, more centralized, Scandinavian-style state, with free university education and health care.
The political divide from Westminster is sharp. Only one Conservative Party member of the British Parliament is from Scotland, so Scots feel that the Conservative-Liberal Democrat coalition led by Mr. Cameron does not really represent them.
At the same time, the British Labour Party, which wins many seats in Scotland, wants to preserve the union, fearing that without Scotland, it would have a much harder time winning a majority in Westminster. So the Labour leader, Ed Miliband, was also said to be preparing a late effort to persuade Scots to stay within the kingdom, promising them that if Labour wins the next election in May, Scotland will get a better deal and a more sympathetic hearing.
Polls now indicate that the vote is neck and neck, after months when the pro-union side remained ahead.
A YouGov poll published in The Sunday Times showed the independence vote inching ahead for the first time. The poll was conducted last week using an opt-in online panel that is not truly a random sampling of potential respondents, but it showed a swing in momentum toward independence, especially among Labour Party voters.
The poll showed pro-independence voters ahead by 51 percent to 49 percent, but that figure excluded those who said they did not know how to vote or who would not give their position.
A YouGov poll released on Sept. 2 had those against independence ahead by six percentage points, down from 14 points in the middle of August and 22 points at the beginning of August.
YouGov’s polls have been considered more weighted toward “no” voters, adding to the anxiety.
“A two-point gap is too small for us to call the outcome,” Peter Kellner, the president of YouGov, wrote on his blog. “But the fact that the contest is too close to call is itself remarkable, as Better Together seemed to have victory in the bag.”
The findings are hardly definitive, and referendums are notoriously hard to predict. Another online poll, commissioned by the pro-independence movement and also not a truly random sampling, was released Sunday by Panelbase and had the “no” side ahead by 52 percent to 48 percent. And the latest average of the various polls, issued Sept. 1 by John Curtice, a professor of politics at the University of Strathclyde, still showed the pro-union lead at 10 points.
If Scotland votes to leave the United Kingdom, independence will not occur before March 2016, leaving lots of time for bitter negotiations that will further preoccupy a Britain that is questioning even its own membership in the European Union and its place in the world.