Jeremy Corbyn has appeared to contradict a call by the Scottish Labour leader for a "new Act of Union" to "save the UK".
Kezia Dugdale made the suggestion last month as she outlined plans for a federal structure for the UK.
Mr Corbyn told BBC Scotland that, like Ms Dugdale, he wanted to set up a constitutional convention to look at arrangements across the country.
But the Labour leader added: "I would not use the words new Act of Union."
Mr Corbyn also said he did not believe a second referendum on Scottish independence was needed.
And he said he did not think devolving immigration controls to Holyrood would be possible, despite calls from a cross-party group of MPs and the Scottish government.
Speaking at an event in London in December, Ms Dugdale said a new Act of Union was needed to "save the UK for generations to come".
She called for a People's Constitutional Convention, modelled on the one set up in Scotland ahead of the creation of the Scottish Parliament, to "re-establish the UK for a new age".
Ms Dugdale added that the convention should be tasked with producing a "new Act of Union which would reaffirm the partnership between our nations and renew it for the future".
Her deputy Alex Rowley has also called for Scottish Labour - which was recently given new powers to set its own policies - to campaign for "home rule within a confederal United Kingdom".
Mr Corbyn said he wanted to look at the constitutional relationship across the UK, but said he "wouldn't use the words new Act of Union".
He added: "What we will be doing is looking at a new constitutional convention for the whole of the UK because there are issues of lack of democracy in parts of our political structure, such as the unelected House of Lords.
"There is a huge issue about regional government across England, and there is a need to have a discussion about the relative powers in Scotland, in Wales and in Northern Ireland of devolved assemblies."
Responding to Mr Corbyn, a spokesman for Scottish Labour said: "Kezia Dugdale has proposed a new Act of Union, to strengthen our union across the whole of the UK for generations to come.
"Being part of the UK protects jobs and the money we need to invest in our public services."
'All at sea'
Scottish Conservative leader Ruth Davidson said: "I never thought I'd find myself agreeing with Jeremy Corbyn.
"But he's quite right to reject the idea of a new act of union; it's clearly little more than a Scottish Labour gimmick, from a party all at sea on the constitution."
The SNP's Ivan McKee MSP said: "This is a complete and utter embarrassment for Scottish Labour, with their position on the constitution fatally undermined by Jeremy Corbyn.
"Just last week Kezia Dugdale claimed that her London leader was backing her all the way, and that he supported her plan 'unequivocally'. But it's now clear that nothing could be further from the truth."
When asked about his views on an independence referendum, Mr Corbyn said "the agreement has been that a second one could be held" if it was wanted by the Scottish Parliament.
But he said it was "much more important" to address the economic issues facing the whole of the UK, as well as the Brexit negotiations with the EU.
He added: "I don't see the need for one, I'm not asking for one, I don't think she (Nicola Sturgeon) should call one."
On suggestions that Scotland and other areas of the UK could have their own immigration policy after Brexit, Mr Corbyn said this would be "very difficult" to implement as it would need "regional borders".
He said: "I have heard the argument in England as well, and in the case of Scotland I would say exactly the same - it has to be a UK-wide decision.
"Clearly Scotland, like other parts of the UK, does benefit from EU workers that come. It does benefit from their skills, and indeed people from Scotland do go to live and work in other parts of Europe. That is actually a good thing."