Labour's Ed Balls has given voters in Scotland a personal guarantee there will be a "big transfer of financial powers" to Holyrood if they reject independence and he becomes the next chancellor.
He made the pledge as he called on Scottish First Minister Alex Salmond to "be clear with Scots about what his plans for independence mean", warning leaving the UK could lead to billions of pounds of spending cuts.
With less than two weeks to go until the referendum, and with polls now indicating the vote could be too close to call, Mr Balls is making a campaigning visit to Aberdeen.
He said that while he understood why "many Scots are frustrated by the callous unfairness of David Cameron's Tory-led Government", he stressed that a Yes vote should not be used as a protest against the coalition Government.
"Voting for independence is not a protest vote. It's a permanent and deeply risky decision to leave the UK," Mr Balls said in an article for the Daily Record newspaper.
With the three main Westminster parties already having outlined plans for further devolution if Scotland remains in the UK - and with a timetable for this expected to be unveiled in days - the Labour MP said: "Let me be clear - a No vote is not a vote for no change.
"It is a vote for more powers for the Scottish Parliament. All the political parties now agree on this.
"And I can give people across Scotland my personal guarantee that as chancellor in the next Labour government, I'll oversee a further and big transfer of financial powers from the Treasury to the Scottish Government.
"That's real powers to improve people's lives, create jobs and make Scotland an even better and more prosperous place to live.
"Scotland can and will have the best of both worlds - with a strong Scottish Parliament backed up by the strength and security of being part of the United Kingdom."
He also insisted that SNP leader Mr Salmond "needs to be clear with Scots" about the impact of a Yes vote on September 18.
But Mr Balls claimed the First Minister had failed to answer key questions over currency and how essential services would be paid for.
"Throughout this campaign there have been no answers from him on the currency, the cost of living, or how he will fund Scotland's NHS and schools," the shadow chancellor said.
"The independent Institute for Fiscal Studies says independence means £6 billion of cuts. That will fall on our public services, schools and the NHS."
He added: "Scotland can have real change. With a No vote next Thursday and the election of a Labour government next May. I don't have a vote. But I really want Scotland to stay."
But SNP MSP Linda Fabiani branded the shadow chancellor's pledge "yet another panicked response from a No campaign in complete disarray".
She said: "As more and more people in Scotland wake up to the opportunities of Yes, the Westminster establishment is growing increasingly desperate.
"The problem for the likes of Ed Balls is that people in Scotland are not impressed with Labour's alliance with the Tories in the No campaign - which is why more and more Labour voters are switching from No to Yes to ensure we never have a Tory government we didn't vote for foisted on the people of Scotland again."