Nigel Farage has said The Queen should make a public statement in support of maintaining the United Kingdom if the Scottish independence battle remains on a knife edge in the days leading up to the September 18 referendum.
Buckingham Palace has stressed that the Queen takes the view that the decision should be left to Scottish voters and that her role was "above politics".
But the Ukip leader claimed that if the United Kingdom was under threat she had a responsibility to speak out.
He said if the opinion polls were finely balanced on Sunday then she should make a statement, highlighting her 1977 Silver Jubilee declaration of support for the union as a precedent for the monarch intervening in constitutional debates.
The Ukip leader told LBC Radio: "My understanding of the constitution is if the kingdom itself, if the United Kingdom itself is under threat, then in many ways you could argue she has a responsibility to say something.
"I can completely understand her wanting to keep out of politics and she has done it absolutely brilliantly over 60 years., she is a globally admired figure.
"I doubt the royal family has ever been more popular over centuries than it currently is.
"But there are times when, if the United Kingdom, over which she is the monarch, is threatened itself, when it might be right to say something.
"That doesn't mean that she has to, but ... let's say we got to this Sunday and let's say it was still 50-50 in the polls, I personally think she should say something, yes."
He added: "If the very future of the United Kingdom itself is threatened she should say something and there is a precedent for this: she did it in 1977, she did it in Westminster Hall when she spoke, her Silver Jubilee speech, when the first modern calls for separation were being heard, and she said very clearly that I am the Queen of the entire United Kingdom.
"So she's said it before and it might be handy if she said it again."
Mr Farage called for a "new constitutional settlement" for the whole United Kingdom following the referendum on Scottish independence.
Mr Farage, who is visiting Glasgow to campaign for a No vote, called for a federal UK with powers devolved to the different parts of the country, and said Scottish MPs should be stripped of the power to vote on English matters at Westminster if "devo-max" goes ahead.
The eurosceptic party leader accused Alex Salmond of offering voters in Scotland "a false prospectus" in the referendum campaign, insisting that a Yes vote will not deliver independence so long as the country remains in the European Union.
No campaigners have urged the Ukip leader - whose last trip north of the border ended with him barricaded inside a pub to escape protesters - to stay away for fear he will alienate traditional Labour supporters.
But he insisted he intends to press ahead with an address to supporters in Glasgow, and said he had "absolutely no intention of being incendiary at all".
Mr Farage told BBC Radio 4's Today programme: "The truth is that Mr Salmond's plan is not for independence. This whole referendum is in danger of going by default.
"Mr Salmond wants Scotland to be part of the EU state. He wants his laws made in Brussels. He's got no chance of renegotiating Scottish fisheries.
"I'm fearful that people have switched to the Yes side, believing this is a noble, bold plan of Mr Salmond's to be a self-governing independent nation, when it's nothing of the kind."
Mr Farage said English people were feeling "ignored" in the independence debate. He argued that the strengthening of the devolution settlement offered by pro-Union parties in the event of a No vote should apply to other parts of the UK, as well as Scotland.
"I am fully in favour of a federal United Kingdom," said Mr Farage. "We need a new constitutional settlement.
"At the moment, the English are feeling rather ignored in all this, because we've been talking about Scotland and Wales and Northern Ireland over the last 17 or 18 years. A new constitutional settlement for the UK, that will suit everyone."
Asked whether Scottish MPs should continue to vote on English matters in Westminster if devo-max goes ahead, Mr Farage said: "No, I don't think it's right at all. I think the overwhelming majority of opinion is that devolution has to be fair to everybody, and right at the moment that's not working."
Former City trader Mr Farage predicted that Scotland's banks will move head offices to London if Yes wins next week's vote, in order to avoid a flight of investors.
He accused Mr Salmond of having "no plan for the currency" and warned that Scotland will be required to sign up to a commitment to join the euro if it wants to be a member of the EU.
"If you're an investor and you've got money invested in a Scottish bank and you wake up next Friday and Scotland has voted Yes and you find yourself in the position where the banking industry in Scotland doesn't have a central bank and a lender of last resort, then you will want to withdraw your funds - which is why the banks will want to come south to London," he said.
Independence for Scotland will leave the UK "diminished globally as a nation", said Mr Farage, adding: "The message that goes out that you've lost a large chunk of your territory and that a 300-year-old union that has achieved astonishing successes for democracy and freedom worldwide has suddenly broken up, that's not a good message."