Ed Miliband has pledged to hand "crucial" new powers to the Scottish Parliament if he is elected prime minister next year.
With less than a year to go to the next UK general election, the Labour leader said he was "determined" to bring his party back to power across the UK.
He used a visit to Dundee to announce a n ew ''contract'' with the people of Scotland - five key pledges that he vowed Labour would deliver in its first batch of legislation.
Central to that is a commitment to introduce a new Scotland Act to devolve further powers north to Holyrood.
It comes after Labour's devolution commission recommended Scotland get more responsibility over incom e tax, including the ability to increase the higher rates - along with powers over housing benefit and the w ork programme.
Mr Miliband said: "We are the people who will give more power to the Scottish Parliament and can have the best of both worlds, which is a Scottish Parliament and the strength and security and social justice that the United Kingdom can bring, if we can elect a Labour government which is what I'm determined to do."
With Scots to decide if the country should stay in the UK or not in September's independence referendum, concerns have been raised that a Yes vote could make it harder for Labour to be elected into power at Westminster.
But the Labour leader, who was visiting the Michelin tyre factory in Dundee, said the issue was "much bigger" than that.
"This isn't about that happens to one party if there's a Yes vote," he said.
"This is about something much bigger than that, which is are we going to be a United Kingdom with Scotland in it.
"Personally I think the rest of the United Kingdom benefits massively from Scotland's presence. If you think about our historic achievements, whether it's the NHS or the minimum wage or workers' rights, they came about in part because of Scottish figures, the Scottish political movement.
"So I believe the rest of the United Kingdom wants Scotland to stay. I also happen to believe its in Scotland's interests to stay. In the end that's a decision for the people of Scotland."
Mr Miliband said he was "very proud" it was a Labour government that had established the Scottish Parliament and added: " I'm very very proud we have committed to extend the powers, especially over crucial issues like tax, housing benefit, the work programme."
Mr Miliband's ''contract'' with Scots also includes a pledge to bring in a 50p rate of income tax for top earners - which could affect 14,000 people north of the border - as well as reintroducing the 10p tax band for low paid workers.
Labour would freeze gas and energy prices, and introduce a jobs guarantee for young people who have been out of work for a year or more, offering them a six-month paid position to help get them back into the employment market.
In addition, zero hours contracts would be banned, with workers being given more rights to demand fixed hours contracts, a move which Labour believes could help at least 90,000 workers in Scotland.
"We are less than a year away from a Labour government that will not only give more powers to the Scottish Parliament, freeze energy bills, get our young people back to work, tackle the exploitation of zero hours contracts and have a fairer tax system," he said.
"That's the social justice agenda, the contract with the people of Scotland that I offer."
He stated: " When Labour came to power in 1997, the first piece of legislation to be discussed under the first Labour Government in eighteen years was the Bill to bring about the referendum on devolution that led to the creation of the Scottish Parliament. We have always been the party of devolution.
"And today I want to reaffirm that belief in devolution, with a promise that Labour will bring forward in its first Queen's Speech a new Scotland Act to put in to law the powers proposed by Scottish Labour's Devolution Commission."
He went on: "Unlike the SNP, our commitment to the people of Scotland is not just about the constitution.
"For too long, people across this country have seen a few people at the top doing well, while the vast majority of hardworking people fail to see any reward. That is what we want to change. People are today trapped by a cost of living crisis that is getting worse under this UK Government.
"This is a crisis that neither the Tory-led UK Government nor the SNP Scottish Government can solve."
Mr Miliband said he was "c onfident" that his party had "the right arguments on how you can better create social justice, and we have the better arguments about the big issues the whole of the United Kingdom faces"
He said that "a ll around the world people are dealing with these issues of inequality and the cost of living crisis" adding that the big " question for the future" was which party would best meet that "generational challenge".
Here he contrasted Labour's "social justice agenda" with the policies of both David Cameron's Tories and Alex Salmond's SNP.
Mr Miliband said: "I offer this social justice agenda. Alex Salmond says his top priority if Scotland goes independent is a 3p cut in corporation tax. David Cameron is never going to address a social justice agenda so I think Labour has the best solutions to this problem.
"Here we are in Michelin, what's their biggest issue - energy prices. That's why our energy price freeze is so important not just for families but businesses as well."
He said: " I want to get on with making the big changes our country needs, because I think that is the way forward to create both the prosperity we need and social justice as well."
He hit out at the Scottish First Minister, claiming he could already be putting in place some of the policies the SNP have pledged to introduce if Scotland leaves the UK, such as plans to transform free childcare.
The Labour leader said: "A lex Salmond is making big promises but he could be delivering those promises now, he's in charge of the Scottish Government, childcare is a devolved issue. So if he's got grand plans for childcare why isn't he actually putting them into practice?
"The Scottish Parliament has powers to act, the SNP has failed to act in a whole range of areas, whether that's on issues like the living wage where they are pushing against Labour's plans on procurement and the living wage, or on a whole range of other issues. They're not even committing in the case of an independent Scotland to bring back the 50p higher rate of income tax.
"I think when people look at that they see where the right social justice choice lies."
While he said he understood the " deep frustration" people have with Mr Cameron's government, he insisted independence was not the solution for Scots.
Mr Miliband said: "I understand the deep frustration people have about this Tory Government, I share it, and that's why I'm determined to get rid of them. That's what we are planning to do in less than a year's time.
"We can build a more just and more equal and more fair Scotland within the United Kingdom. That's why we're laying out these detailed plans on energy, on jobs for young people, on a whole host of issues that matter like zero hours contracts, like a fairer tax system, and with greater powers for the Scottish Parliament to make that happen."
After Mr Cameron indicated he would stay on as Prime Minister if Scots vote Yes in September, Mr Miliband said the issue of independence was "not about" individual politicians.
He insisted: "T his isn't about what happens to David Cameron or any individual, this is much bigger than that. This is about what kind of country is the United Kingdom going to be and what is best for Scotland.
"It is not about one individual, it's about the future of a country, and it's about the decision, the big, big, momentous decision that the people of Scotland have to make."
But the SNP claimed Mr Miliband had "nothing to offer" in the debate over Scotland's future, and insisted only independence would guarantee more powers for Holyrood.
SNP MSP Stewart Maxwell hit out: "Ed Miliband's speech today had nothing to offer the debate on Scotland's future - the fact remains that only a Yes vote guarantees more powers.
"And only a Yes vote in September guarantees we always get the governments we vote for and never again have to put up with Westminster Tory governments we didn't elect."
Mr Maxwell said Labour's devolution commission "completely failed to offer substantial new powers" adding that "academics and experts have criticised the lack of vision in Labour's proposals".
He contrasted this with the Scottish Government's white paper on independence, which he said "sets out a detailed vision of what an independent Scotland will look like - setting out how we can work toward creating a fairer Scotland, closing the gap between rich and poor that only grew wider under the last Labour government".
Mr Maxwell said: "The people of Scotland will not be fooled by Labour's promises of further powers. In 1979 we were promised 'a better form of Devolution' if we voted No - but in reality all we got was 18 years of Tory government.
"With a Yes vote in September we will no longer have to put up with Tory policies like the unfair bedroom tax and dumping Trident nuclear weapons on the Clyde.
"In contrast Labour can't even guarantee Ed Miliband will become Prime Minister - never mind the powers Scotland needs to thrive."