LABOUR and the Tories were at loggerheads last night over the role David Cameron would play in the anti-independence campaign after Cabinet sources insisted the Prime Minister would lead the pro-Union forces in Scotland ahead of a referendum.

"He is the Prime Minister. People would expect him to lead it," a Conservative Cabinet minister told The Herald.

When it was suggested Mr Cameron might be seen as an English Tory interloper by many Scots, he replied: "It will be difficult but he is the Prime Minister."

Last night, some senior Labour sources reacted with horror at the thought of Mr Cameron heading up the anti-independence campaign.

One told The Herald: "Salmond wants us to be seen as an army led by David Cameron but the idea that the Prime Minister will lead the campaign is bizarre. The best advice for Cameron is to butt out."

Ian Davidson, the Labour MP for Glasgow South West, noted: "Anyone who believes Cameron is the best person to lead the anti-separation forces has not much understanding of the politics of Scotland."

Jim Murphy, the Shadow Defence Secretary, has made clear he will not share a platform with the PM during the referendum campaign and it was confirmed by party sources yesterday that Johann Lamont, the new Scottish Labour leader, would take the same line. "Being seen with them would be toxic," declared one Labour MP.

However, Tom Harris, the former Labour transport minister who represents Glasgow South, made clear Labour politicians should not be "precious" about whom they shared a platform with so long as it furthered the cause of opposing Scottish independence. "We should focus on saving the Union," he said.

Mr Harris was also less hostile about the PM's involvement, saying Mr Cameron could lead a Conservative anti-independence campaign as there were many people in Scotland who did not vote Tory but who would vote no to independence.

Thomas Docherty, the Labour MP for Dunfermline and West Fife, was also less critical of Mr Cameron becoming involved, saying: "If we say the PM can't play a role in the campaign then

we fall into the SNP trap." He added: "Speaking diplomatically, the PM has limited popularity in Scotland but he does appeal to some people."

Labour sources explained they envisaged parallel anti-independence campaigns by the pro-Union parties; one suggested Anas Sarwar, the Glasgow MP just voted Scottish Labour's deputy leader, would be named in the New Year as the party's campaign co-ordinator.

Yet, at some point there will have to be an official no-to-independence campaign. Already Labour grandees John Reid and Alistair Darling have ruled themselves out as its leader, as has Mr Murphy.

The cross-party talks on forming a pro-Unionist campaign stalled because of the delay in Labour finding a successor to Iain Gray. The anti-independence lobby now hopes with Ms Lamont in place the formation of an official pro-Union campaign will move apace next year

"2012 is the year we have to establish the arguments and control the agenda," said Mr Harris.

Mr Davidson, however, appeared more relaxed, saying there had to be "more forensic examination" of the potential consequences of independence and ensuring the poll was a fair one.

The importance of the constitutional battle ahead was underlined this week by Sir Gus O'Donnell, the outgoing head of the civil service, who said it was one of the "enormous challenges" facing the Coalition Government.

Meantime, one Labour backbencher insisted the option of Westminster staging an independence referendum was not, as has been reported, ruled out.

He stressed the referendum had to be overseen by the Electoral Commission and the poll must take place by 2014.

The backbencher said if Mr Salmond did not agree to these conditions, then the so-called "Westminster option" would be engaged.