David Cameron has insisted that a TV showdown with Alex Salmond in the run-up to next year's independence referendum would be a "phoney debate".

Nationalists have called on the Prime Minister to take part in a television debate with the First Minister ahead of the key vote on Scotland's future - due to be held on September 18 2014.

But Mr Cameron dismissed this as "diversion tactics" from the SNP, who he claimed were losing the argument on independence.

The Prime Minister spoke out on the issue during a visit to Scotland in which he went out on one of the Trident nuclear submarines, before visiting a factory in Glasgow which supplies the armed forces with high-tech equipment.

Mr Cameron said: "The debate is about the people of Scotland making a decision, so the proper debate is for those in Scotland who want to stay in the United Kingdom to debate with those in Scotland who want to leave the United Kingdom.

"That's the debate that needs to take place, rather than a rather phoney debate, as it would be, between the Prime Minister of the United Kingdom and the First Minister of Scotland."

Scottish Deputy First Minister Nicola Sturgeon said last month that Mr Cameron had "a duty as the most senior politician in the anti-independence movement" to debate the country's future with Mr Salmond.

The Prime Minister insisted today: "I've made clear my views, but Alex Salmond wants to have a diversion tactic."

He added he was "certainly not" scared of going head to head with the Scottish National Party leader, but claimed: "He's losing the argument so he wants to change the question. Because he's losing the argument he suddenly now wants a debate with the British Prime Minister.

"He spends most of the time telling me to butt out of Scotland's business, now he seems to want me to butt back in again. I think that's because Alex is losing the argument."

Mr Cameron said the debate over Scotland's future "should be had at a very serious level" examining the issues that affect people north of the border, such as defence, jobs, the economy and currency.

"But also let's make it a debate of the heart as well as the head," he argued.

"I actually think that Scotland as part of the United Kingdom has been part of this incredible alliance, this incredible family, that has done great things in our world, and can continue to do so.

"That's the debate that needs to be held. But it's not a debate between me and Alex Salmond, it's a debate for the Scottish people to have between those who want to leave and those who want to stay.

"And then it's the Scottish people who get to decide. I don't have a vote in this, but I passionately hope Scotland will stay with the United Kingdom."

But Ms Sturgeon attacked the Prime Minister for focusing on defence and Trident on his visit.

She insisted Scots were "utterly opposed to nuclear weapons being based here" and said only an independent Scotland would be able to get remove Trident from its base on the Clyde.

Ms Sturgeon said the Prime Minister's visit "comes in the same week that his Government has introduced the Bedroom Tax - a deeply unfair policy that will adversely affect thousands of families across Scotland".

She stated: "That is the kind of issue that is concerning people the length and breadth of this country - and for the Prime Minister to focus his visit on the case for nuclear weapons shows that his priorities are wrong and that he is out of touch with the needs and aspirations of ordinary families in Scotland."