David Cameron and Ed Miliband joined forces today in pledging to fight to keep Scotland in the United Kingdom, as Scottish First Minister Alex Salmond told Westminster politicians to "butt out" of the debate.
The leaders of both major Westminster parties called on Mr Salmond's Scottish National Party to engage in talks over the coming weeks to pave the way for a referendum on independence.
Mr Cameron said he "passionately" believed in preserving the Union, and taunted the SNP for seeking to delay a ballot, telling MPs: "Let's have the debate and let's keep our country together."
But Mr Salmond dismissed the UK Government's argument that Scotland's Parliament has no legal power to mount its own referendum on independence.
He will unveil his own proposals before the end of this month for a referendum in the autumn of 2014.
The First Minister's plans, set for publication in the week beginning January 23, look set to put Edinburgh on collision course with London.
Mr Salmond's office said they will include a vote for 16 and 17-year-olds and may offer voters a third "devo-max" option, under which Scotland would stay in the Union but gain greater self-determination on financial issues. Both of these features were ruled out in UK Government proposals put forward by Scotland Secretary Michael Moore.
Mr Moore called on the Scottish administration to work with the UK Government over the coming weeks to agree arrangements for a "clear, legal and decisive" referendum, which could be held within 18 months.
And Mr Cameron's spokesman indicated that this could involve talks between the Prime Minister and Mr Salmond.
The spokesman said he expected negotiations over the referendum - and the independence battle itself - to involve senior politicians from all sides of the debate, including a number of Government ministers.
He did not rule out the possibility that Mr Cameron could appear alongside Mr Miliband to argue the case for the Union, after the Prime Minister said he agreed "100%" with the Labour leader on the benefits of preserving the United Kingdom.
In a rare display of unity at Prime Minister's Questions in the House of Commons, Mr Cameron said: "This is one area where (Mr Miliband) and I are going to be 100% in agreement.
"I passionately believe in the future of our United Kingdom. I passionately believe we are stronger together rather than breaking apart.
"Frankly, I am sad we are even having this debate, because I support the United Kingdom so strongly, but we have to respect the fact that Scotland voted for a separatist party at the Scottish parliamentary elections."
Mr Miliband said: "We on this side of the House believe the United Kingdom benefits the people of Scotland and the people of the rest of the United Kingdom in equal measure.
"We are stronger together and weaker apart."
Mr Cameron said he was "looking forward" to the debate, and mocked the SNP for seeking to delay the poll until 2014, telling MPs: "I sometimes think when I listen to them that it is not a referendum they want, it's a never-endum."
Asked whether the PM would be willing to share a platform with Mr Miliband, Mr Cameron's official spokesman said: "The Prime Minister is keen to make the case for the Union and you saw at Prime Minister's Questions that he is keen to work with the opposition on this."
Mr Salmond said the Prime Minister should "butt out" of an issue for Scottish people and said his intervention had provoked a "huge adverse reaction... to the Thatcheresque idea that Downing Street knows best".
He had "no objection" to Mr Moore's proposals for Westminster to help Holyrood stage a referendum, but said there was "plenty of legal authority" for the Scottish Government to mount its own ballot without assistance from south of the border.
Mr Salmond told BBC Radio Scotland's Good Morning Scotland programme: "The objection, of course, is that the Prime Minister has started to put all sorts of London-based strings on."
He said: "Our conditions are quite clear: this must be a referendum built and run in Scotland, accountable to the Scottish Parliament. It has to be run fairly and transparently, of course, but we won't accept unreasonable conditions placed by London on how Scotland should run the poll."
Mr Salmond's office confirmed the Scottish Government's position that 16 and 17-year-olds on the electoral register in Scotland should be "fully entitled to vote" in any referendum. Asked if voters would be offered a third "devo-max" option, a spokesman said: "We're not for closing down choice."