Ed Miliband has called on Scots to reject independence and join his "mission" to bring social and economic change to the UK.
The Labour leader said there was "deep discontent" across the country, with people "crying out" for economic change.
He said that "in the face of an economy that doesn't work and a politics that is broken, some people might be tempted to vote Yes" to Scottish independence in September's referendum
Mr Miliband insisted: "There is another, better way of changing things. Our mission - economic and social change."
With less than three months to go until voters in Scotland decide the future of the UK, Mr Miliband told an audience in Edinburgh: "B y saying No in the referendum, the people of Scotland can say Yes to the campaign to change Britain as a whole.
" To change our economy and the way we are governed."
Labour has already pledged to deliver more devolution to Scotland - including new income tax powers - if the Scots opt to stay in the United Kingdom.
But he said that devolution "must not stop in Scotland".
Mr Miliband said the Welsh Assembly should be put "on the same legal footing as the Scottish Parliament".
He also said his party would set out its plans next week to "restore the great cities of our country as the great powerhouses of our economy".
The Labour leader pledged: "We will be better able to change Scotland and the whole of the UK if we stay together."
Mr Miliband said the " question for this and future generation of Scots" was how best could the nation's "traditions of equality and solidarity" be put into practice
He asked if this was best achieved "in an independent Scotland, apart from the rest of the UK, or at the heart of a movement that goes beyond the border and changes all of Britain?".
He told people in Edinburgh: " That is your decision. But it is one I care passionately about.
"I believe the best choice for Scotland is to help us change the whole of the United Kingdom. A nd to do it over the next year, by electing a Labour government that can make that change happen."
He said change was possible, arguing: "It doesn't have to be this way."
The Labour leader said: " By voting No, the people of Scotland won't be voting for things to stay the same. You won't be supporting the status quo.
"By voting No, you can say Yes to the biggest progressive change for a generation."
He continued: " My appeal to the people of Scotland is to be part of our mission to change Britain.
"Scotland has always thought big. And the way to think big in the 21st century is by changing the United Kingdom.
"Changing our economy to make it more equal. Changing our society to make it more fair. And changing our politics: to reform the British state so it works for people in every part of our country."
Mr Miliband pledged: "I know people want big change and that is what we will deliver."
The Labour leader said if his party was returned to government after next year's general election, Labour would guarantee young people who had been out of work for more than 12 months a job paying at least the minimum wage.
He also vowed to raise the minimum wage by "more than average earnings", ban "exploitative" zero-hours contracts and bring back the 50p rate of income tax for top earners.
Mr Miliband hailed those as examples of "real change that will happen under a Labour government".
He said his party's mission was "changing the state".
He stressed: "We offer change to the way the British state works, not more of the same.
"By saying No in the referendum, the people of Scotland can say Yes to the campaign to change Britain as a whole.
"To change our economy and the way we are governed.
"And we will be better able to change Scotland and the whole of the UK if we stay together."