Former prime minister Gordon Brown has called for a "fairer deal" for Scotland, but urged people not to break up the union.
In a constituency speech, Mr Brown stated that "enhanced devolution within the union" would make the Scottish Parliament more powerful.
As Labour's devolution commission continues to consult on its findings, with final recommendations to be decided in time for the independence referendum, Mr Brown underlined the importance of delivering major constitutional change.
The Scottish Parliament should be made irreversible, with "maximum devolution of powers in training, transport, health, the Crown Estates Commission and the running of elections", he said.
Mr Brown believes Scotland would be strengthened by proposed constitutional reforms to create a "union for social justice" in which the UK can pool and share resources.
The SNP said Mr Brown is "unwittingly making the case for a Yes vote" because he did not deliver further devolution during his time at the top of the UK Government.
The Kirkcaldy and Cowdenbeath MP was speaking as Labour's devolution commission continues to consult on its findings, with final recommendations to be decided in time for the independence referendum.
He said: "I am of the view that the party that first created a powerful Scottish Parliament is best-placed to strengthen devolution and to create a stronger Scottish Parliament in a stronger UK.
"We can show how with our reforms, to be implemented by Labour administrations in Westminster and in Edinburgh, we can address some of the greatest social and economic challenges a future Scotland faces."
Mr Brown also attacked the SNP over funding for their plans for an independent Scotland.
He said: "First, they calculate oil and gas revenues as at least £6.8 billion in 2016-2017 when all formal and independent forecasts suggest the correct figure is likely to be around £3.5 billion, leaving a £3.3 billion shortfall. To make this up requires a rise in income tax of 10p.
"Second, they have failed to calculate the cost of European Union membership without the British rebate, which Scotland would not benefit from.
"In consequence, Scotland's net membership costs could be as high as £500 million that the SNP have not budgeted for. Scotland may even have to contribute to the remaining UK rebate like all other member states.
"Third, while the SNP have a working party on the 'affordability' of pensions, Scotland receives proportionately more spending on pensioners than the rest of the UK and more in incapacity benefit.
"With the rising number of pensioners in future years, Scotland will receive an even greater dividend from its membership of the United Kingdom."
He added: "It is right to set out a Labour vision of the future: the alternative to separation that shows how enhanced devolution within the union offers the Scottish people a fairer deal and a better dividend."
Deputy Scottish Labour leader Anas Sarwar also spoke at the event in Lochgelly.
He said: "There are those whose talents lie in re-writing history, where for them airbrushing the successes of the Labour movement across the UK is an everyday ambition.
"Quite happy to say in one breath that the UK has never helped to achieve social justice then on the other saying we need independence to protect the NHS and the Welfare State. Institutions thought up by, created by and delivered by the Labour movement right across the UK.
"And there is a reason for that. It's a deliberate attempt to con Labour voters into thinking that no change, or no good, can ever come through a Union between Scotland and the rest of the UK and that only a vote for independence can bring change. Well, they're wrong."
SNP treasury spokesman Stewart Hosie said: "Gordon Brown's speech sounds as if he has no idea what is going on in the real world - the reality is that the welfare state is being dismantled by Westminster.
"George Osborne has announced £12 billion more welfare cuts, hitting the the most vulnerable people the hardest, and Labour are engaged in a Dutch auction with the Tories about who will cut the most.
"Scotland's finances are stronger than the UK's as a whole - we generate 9.9% of UK tax revenues but get just 9.3% of the spending, and therefore a smaller share of our national wealth is spent on welfare, which means that pensions and benefits are more affordable for Scotland.
"A majority of people in Scotland believe that decisions on tax and welfare should be taken in the Scottish Parliament, not Westminster - and the only way to achieve that is to vote Yes for an independent Scotland in September."Suggest a correction