Ed Miliband has set out his case for keeping Scotland within the Union in a speech in Glasgow on Monday
The Labour leader talked of the "progressive" argument for rejecting independence in what the party billed as a major speech on the constitution.
Miliband's speech comes a day after Alex Salmond rejected suggestions that voters should be asked in the referendum on Scottish independence if they want to leave the United Kingdom.
The Scottish First Minister said introducing the UK into the question would "confuse the issue" because the country would retain the Queen as head of state after breaking the political union.
In acknowledging Labour's defeat to the SNP at last May's Scottish Parliament election, Miliband said: "I come here with humility about the scale of challenge for Labour - nine months after we lost the Scottish elections."
Sharing the stage with the new Scottish Labour leader Johann Lamont, Miliband said: "I have no doubt, even as we speak, that the SNP are getting ready to say how dare I, as someone born and living in England, come here and join this argument. And when they ask, what has it got to do with me, let me address this head-on.
"Not just as leader of the Labour Party, but on the basis of my personal history, as someone who has a deep reason to appreciate the strength of the United Kingdom."
Ed Miliband talked about how his father trained for the Royal Navy aboard HMS Valorous on the Firth of Forth, and insisted that: "if the people of Scotland decide to separate, as they can, it would not affect Scotland alone. It will affect all of us in the four nations of this country."
He went on to day: "I support Scotland as part of the United Kingdom, not because I think Scotland is too poor or too weak to break away. But for a profoundly different reason.
"Because I believe that Scotland as part of the United Kingdom is better for the working people of Scotland, and better for the working people of the United Kingdom as a whole."
His intervention comes five days after Scotland's First Minister Alex Salmond launched his consultation on the staging of the independence referendum, which he wants to hold in autumn 2014.
At an international press conference at Edinburgh Castle, and earlier inside the Scottish Parliament, Mr Salmond set out his plan to end the three-centuries union with England.
He proposes that voters are asked: do you agree that Scotland should be an independent country?
Salmond used a lecture in London last week to describe an independent Scotland as a "beacon for progressive opinion" for those south of the border.
Later on Monday a campaign will be launched in Edinburgh to seek wider views on the future of the constitution, including the potential for a second question on extra powers short of full independence.
The movement brings together a coalition of organisations, including the Scottish Council for Voluntary Organisations, and is backed by former Labour first minister Henry McLeish.
Salmond has proposed a straight yes-no question but left room for further options for enhance devolution, so-called devo-max.
Commenting on Miliband's expected comments, SNP MSP Bill Kidd said: "The attitude of UK leaders like Mr Miliband to Scotland’s ambitions to run our own affairs and take responsibility for our own decisions is just one of the factors holding them back in Scotland."