The leaders of the three main parties at Westminster have signed a pledge to devolve more powers to Scotland, if Scots reject independence.
The pledge, which appears on the front of the Daily Record newspaper, has been signed by David Cameron, Ed Miliband and Nick Clegg.
It has three parts and also commits to preserving the Barnett funding formula.
The "Yes" campaign has argued the only guarantee of more powers is a vote for independence.
The first part of the agreement promises "extensive new powers" for the Scottish Parliament "delivered by the process and to the timetable agreed" by the three parties.
The second says the leaders agree that "the UK exists to ensure opportunity and security for all by sharing our resources equitably".
The third "categorically states" that the final say on funding for the NHS will lie with the Scottish government "because of the continuation of the Barnett allocation for resources, and the powers of the Scottish Parliament to raise revenue".
The Barnett formula is the method used to determine the distribution of public spending around the UK.
The shadow foreign secretary denied the powers pledge had come too late in the referendum debate.
Speaking on BBC Breakfast, Douglas Alexander said: "Here in Scotland, we have been talking about these powers for many months. What we are saying today is we can have the best of both worlds. We can have a stronger Scottish parliament but with the strength, stability and security of the United Kingdom.
"That pledge, that vow that we can have faster, safer, better change is actually a vision around which Scotland can unite."
He added: "I don't think there's any embarrassment about placing policies on the front page of papers with just days to go. I think the 'Yes' campaign are struggling.
"The economic risks suddenly became very real last week, and at the same time we are offering what I believe most of use here in Scotland which is faster, safer and better change."
Mr Alexander dismissed "Yes" campaign claims that independence is the only way to get the government Scotland votes for.
- Social media giant Facebook has seen 10 million interactions concerning the Scottish independence referendum in a five-week period.
- Confidential papers passed to the BBC suggest a radical cost-saving plan will be implemented in the Scottish NHS after the referendum.
- Deputy First Minister Nicola Sturgeon and Finance Secretary John Swinney will meet apprentices at an engineering firm in Renfrew. They will argue that independence will allow Scotland's economy to grow.
- Labour leader Ed Miliband and former Prime Minister Gordon Brown will be on the campaign trail for Better Together. Mr Miliband will say a vote for independence will be "an irreversible decision" and "a risk to jobs".
Also speaking on BBC Breakfast, Deputy First Minister Nicola Sturgeon said: "If there was a serious intention to deliver more powers, why hasn't that happened before now?
"Tory MPs, including Christopher Chope, have already said they would block more powers. If we vote 'No', there are no guarantees at all."
She added: "They [the pro-Union parties] are treating voters in Scotland with contempt."
Asked how a "Yes" vote guarantee "better lives" for people in Scotland, she replied: "Independence is not a magic want, but it is a massive opportunity.
"We can make life better, not overnight, but over time."
The pledges were first outlined by the former prime minister, Gordon Brown, on Monday.
The way the announcement has been made echoes the original pledge by Mr Cameron, Mr Clegg and Mr Miliband of greater devolution for Scotland. That promise was also first announced by Mr Brown.
Speaking on the BBC's Newsnight programme, the prime minister said more powers had always been part of his plan.
"I always said right from the start of this campaign, if Scotland voted 'No' to separation, the rest of the United Kingdom would say 'Yes' to further devolution," said Mr Cameron.
"If Scotland wants more devolution - and I think Scotland should have more devolution - you have to answer the prior question 'Do you want to stay in the United Kingdom?'
"And of course that wasn't just my view; that was the view of the leaders of other United Kingdom parties who all thought it was important. Let's settle the question of separation and then look at devolution."
Mr Cameron also said the question of whether Scottish MPs at Westminster should vote on laws that do not affect them will get "more pressing" if there is further devolution to Scotland.
On the penultimate day of campaigning ahead of Thursday's referendum, the "Yes" side will focus on jobs and the NHS, while the "No" side will promise change and a "better Britain".
Deputy First Minister Nicola Sturgeon and Finance Secretary John Swinney will meet apprentices at an engineering firm in Renfrew. They will argue that independence will allow Scotland's economy to grow, creating jobs and opportunities.
Ms Sturgeon will then join carers to talk about the NHS and welfare reform.
Ahead of the visit, she said: "In just two days' time, polling stations will open and voters across the country will hold Scotland's future in their hands. Independence is our opportunity to build a better future - creating jobs and protecting our NHS.
"Only a 'Yes' vote will ensure we have full powers over job creation - enabling us to create more and better jobs across the country. So instead of almost 40,000 young people leaving Scotland each year as is currently the case, there will be more opportunities for our young people here at home.
"As part of the UK, our NHS budget faces knock-on impacts of the privatisation, cuts and charging agenda that is ripping the health service south of the border to bits. With a 'Yes' vote we can ensure our NHS is protected for future generations by enshrining it in our written constitution."
Labour leader Ed Miliband and former Prime Minister Gordon Brown will be out on the campaign trail for Better Together.
Mr Miliband will say a vote for independence will be "an irreversible decision" and "a risk to jobs, the economy and the NHS" and he will highlight the offer of more powers from the pro-Union parties.
He is expected to say: "In the next 48 hours Scotland faces a historic decision which will shape its future and the whole of the UK's future for centuries to come.
"The will of the people of Scotland for economic and political change has been heard and we will deliver.
"Change is coming with more powers on tax and welfare for the Scottish Parliament.
"We will change the British state too, the House of Lords and the way we work together across our nations. I ask the people of Scotland to lead that change of our whole British constitution."
On Monday, Alex Salmond met with pro-independence business leaders in Edinburgh, while Mr Cameron made a speech in Aberdeen promising further devolution.
The first minister argued that an independent Scotland would be prosperous and fair, and he criticised the "scaremongering" of the "No" campaign.
The prime minister said he wanted Scots to stay part of the United Kingdom with his head, his heart and his soul.