Story highlights Leaders of the three main UK parties give Scotland a joint promise of extra powers

Voters will go to the polls Thursday to decide on independence for Scotland

Prime Minister David Cameron begs Scotland to stay, says it's a "once-and-for-all decision"

Pro-independence chief Alex Salmond says Scottish people want their future in their hands

The three main UK party leaders gave a joint promise Tuesday that Scotland will get extra powers if it opts to stay part of the United Kingdom in a landmark vote in two days' time.

Opinion polls have put the pro-independence and pro-union camps neck-and-neck in the run-up to Thursday's referendum.

With the survival of the 300-year-old union of England, Scotland and Wales as Great Britain on a knife-edge, UK Prime Minister David Cameron of the Conservatives, coalition partner Nick Clegg of the Liberal Democrats and Labour leader Ed Miliband have joined forces to beg Scotland to stay.

In a pledge published on the front page of Scotland's Daily Record newspaper, the three leaders say that if Scotland's voters reject independence, work to devolve "extensive new powers" from the central government in Westminster will start Friday.

The issue of spending on social welfare and health care, through the National Health Service, has been central to the pro-independence campaign. Questions over the economy and taxation have also been key.

In their promise, the leaders say "we can state categorically that the final say on how much is spent on the NHS will be a matter for the Scottish Parliament."

They conclude, "People want to see change. A No vote will deliver faster, safer and better change than separation."

'Once-in-a-lifetime opportunity'

With a huge turnout expected for the referendum, both sides are desperate to win over the undecided voters who could hold the future of the United Kingdom in their hands.

In a speech Monday in Aberdeen, Scotland, Cameron made an emotional plea for Scotland not to "end the United Kingdom as we know it" and warned that there was no turning back if voters decide on independence.

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Photos: Photos: Scotland divided Photos: Photos: Scotland divided Scotland divided over independence – Former British Prime Minister Gordon Brown speaks at a rally against Scottish independence Wednesday, September 17, in Glasgow, Scotland. Scots will head to the polls on Thursday, September 18, to vote on a referendum that could end Scotland's 307-year union with England and Wales as Great Britain -- and launch it into the world as an independent nation. Hide Caption 1 of 26 Photos: Photos: Scotland divided Scotland divided over independence – People gather during a pro-independence rally in Glasgow on September 17. Hide Caption 2 of 26 Photos: Photos: Scotland divided Scotland divided over independence – A bakery in Edinburgh, Scotland, reveals the results of a tasty straw poll it has held since March 7. It has been selling cupcakes with the Scottish flag (yes to independence), the British flag (no to independence) and a question mark (undecided). Hide Caption 3 of 26 Photos: Photos: Scotland divided Scotland divided over independence – People hold "no" banners during a pro-union rally in Edinburgh on Tuesday, September 16. Hide Caption 4 of 26 Photos: Photos: Scotland divided Scotland divided over independence – A spray-painted "yes" for independence is seen on the cliffs in Eyemouth, which is on Scotland's North Sea coast, on September 16. Hide Caption 5 of 26 Photos: Photos: Scotland divided Scotland divided over independence – A pro-independence campaigner canvasses voters in Edinburgh on September 16. Hide Caption 6 of 26 Photos: Photos: Scotland divided Scotland divided over independence – Darren Brander shows his support for Scottish independence outside his home in Glasgow on Monday, September 15. Hide Caption 7 of 26 Photos: Photos: Scotland divided Scotland divided over independence – A man hangs a banner on a "Better Together" campaign office in Glasgow on September 15. Hide Caption 8 of 26 Photos: Photos: Scotland divided Scotland divided over independence – Independence supporters take a selfie ahead of a concert in Edinburgh on Sunday, September 14. Hide Caption 9 of 26 Photos: Photos: Scotland divided Scotland divided over independence – Pro-union supporters gather during a rally in Edinburgh on September 14. Hide Caption 10 of 26 Photos: Photos: Scotland divided Scotland divided over independence – An independence supporter takes part in a march to the BBC Scotland offices in Glasgow on September 14. Hide Caption 11 of 26 Photos: Photos: Scotland divided Scotland divided over independence – A dog wears a "no thanks" badge during a pro-union rally in Edinburgh on September 14. Hide Caption 12 of 26 Photos: Photos: Scotland divided Scotland divided over independence – Members of the "Generation Yes" youth organization campaign in Glasgow on Saturday, September 13. Hide Caption 13 of 26 Photos: Photos: Scotland divided Scotland divided over independence – A member of the Grand Orange Lodge of Scotland prepares for a march in Edinburgh on September 13. The Orange Order is a conservative British unionist organization. Hide Caption 14 of 26 Photos: Photos: Scotland divided Scotland divided over independence – People campaigning for both sides of the independence referendum clash in Glasgow on Friday, September 12. Hide Caption 15 of 26 Photos: Photos: Scotland divided Scotland divided over independence – An independence supporter rides a decorated bicycle during a rally in Glasgow on September 12. Hide Caption 16 of 26 Photos: Photos: Scotland divided Scotland divided over independence – People in Edinburgh, Scotland, hold leaflets September 12 campaigning against independence. Hide Caption 17 of 26 Photos: Photos: Scotland divided Scotland divided over independence – Scotland's First Minister Alex Salmond arrives to speak at a news conference in Edinburgh on September 12. Salmond has been a vocal proponent of independence. Hide Caption 18 of 26 Photos: Photos: Scotland divided Scotland divided over independence – First-time voters who are 16 years old depart BBC's Big, Big Debate in Glasgow on Thursday, September 11. Thousands of young voters from all over Scotland attended the debate, where they got a chance to quiz a panel of politicians on the referendum. Hide Caption 19 of 26 Photos: Photos: Scotland divided Scotland divided over independence – British Deputy Prime Minister Nick Clegg speaks to members of the media in Selkirk, Scotland, on Wednesday, September 10. He and British Prime Minister David Cameron were in Scotland to make the case for voting "no" to independence. Hide Caption 20 of 26 Photos: Photos: Scotland divided Scotland divided over independence – "No" is written on a "yes" campaign placard in Selkirk on September 10. Hide Caption 21 of 26 Photos: Photos: Scotland divided Scotland divided over independence – British Labour Party politician Alistair Darling campaigns for a ''no'' vote in Glasgow on September 10. Hide Caption 22 of 26 Photos: Photos: Scotland divided Scotland divided over independence – A woman looks from a window next to a "yes" campaign sign in Edinburgh on Tuesday, September 9. Hide Caption 23 of 26 Photos: Photos: Scotland divided Scotland divided over independence – A man attends a pro-union "Better Together" rally in Edinburgh on Monday, September 8. Hide Caption 24 of 26 Photos: Photos: Scotland divided Scotland divided over independence – Members of the "English Scots for Yes" campaign hold a tea party Sunday, September 7, in Berwick-upon-Tweed, England, which is near the Scottish border. Hide Caption 25 of 26 Photos: Photos: Scotland divided Scotland divided over independence – The national flag of the United Kingdom, left, and the national flag of Scotland fly above buildings on Downing Street in London. Hide Caption 26 of 26

"This is a decision that could break up our family of nations, and rip Scotland from the rest of the UK," he said.

"And we must be very clear. There's no going back from this. No rerun. This is a once-and-for-all decision.

"If Scotland votes yes, the UK will split, and we will go our separate ways forever."

Cameron said he spoke for millions of people across the United Kingdom -- including in Scotland -- who would be "heartbroken" to see Scotland leave, and he warned that it would be a "painful divorce."

The "Yes Scotland" campaign, led by Scotland's First Minister Alex Salmond, says that only by voting for independence can Scotland be sure that it will have full control of its own taxation and spending.

Salmond, speaking in Edinburgh on Monday, called the vote a "once-in-a-lifetime opportunity" for Scotland.

"Nobody goes back" from independence, he said. "The evidence is that more and more people in Scotland are wanting to put Scotland's future into Scotland's hands."

The yes/no referendum question will ask voters, "Should Scotland be an independent country?"

White House: We want a strong, robust UK

White House spokesman Josh Earnest weighed into the debate Monday, echoing remarks previously made by U.S. President Barack Obama.

"This is a decision for the people of Scotland to make," he said. "We certainly respect the right of individual Scots to make a decision about the -- along these lines.

"But, you know, as the President himself said, we have an interest in seeing the United Kingdom remain strong, robust, united and an effective partner."

Many unanswered questions revolve around defense if Scotland votes for independence. The Scottish National Party, headed by Salmond, has said it wants to remove the UK nuclear submarine fleet from Scottish waters as soon as possible.

'A fool's errand'

The debate has polarized opinion in the business world, as well as in political circles.

The Financial Times published a full-page editorial last week headlined, "The case for union is overwhelming. The path of separation is a fool's errand."

Speaking to CNN's Christiane Amanpour this week, editor Lionel Barber said, "We really believe that it would be very dangerous and highly risky in terms of economics. ... We believe that the 'Yes' campaign has not answered important questions -- notably: What will the currency be? We don't know."

The three main parties in Westminster have said that an independent Scotland would not be able to keep the pound. There are also unanswered questions about tax increases and the fiscal implications of independence, Barber said.

While the newspaper does not deny that Scotland has the right to choose independence, he said, "We are just pointing to the dangers of uncertainty, the price of uncertainty, which will affect ordinary people."

Salmond, who met with business leaders in Edinburgh, said the "impressive array" of business leaders who back the independence campaign "is a very strong point in the 'Yes' campaign."

"These are the people looking forward to the realization that Scotland can have a more prosperous economy but also wants a more just society. And their voice and that combination of things is something that will carry a good deal of weight off the ballot sheet."