DAVID Cameron has brushed aside SNP demands to guarantee Scotland will not be forced out of the European Union against its will as he said the country had opted to remain part of "one United Kingdom" by voting against independence in the 2014 referendum.

The Prime Minister also told MPs that he was unsure whether there would be “many circumstances where the Spanish would ever let an independent Scotland back into the EU” with the Madrid government facing its own breakaway threat from Catalonia.

The rejection of the Nationalist demand came as the Conservative leader finally caved into pressure to allow Cabinet ministers to campaign on either side of the EU referendum debate once a deal with the Brussels bloc is negotiated.

The Herald's View: The danger of high-handedness from the Prime Minister

In heated Commons exchanges, Angus Robertson, the SNP leader at Westminster, asked Mr Cameron if he would finally “give a guarantee that, if Scotland votes to remain within the EU, it will stay”.

As some Tory MPs groaned their displeasure, Mr Robertson insisted: “The people of Scotland want to know if they will be taken out of the European Union against their will.”

To cheers from Conservatives, the Prime Minister replied: “Scotland had a referendum on whether to remain part of the United Kingdom. The Scottish First Minister (Alex Salmond), now an MP, signed with me the Edinburgh Agreement, that said both sides had to respect the outcome of that referendum. That is the only answer he needs.”

First Minister Nicola Sturgeon has repeatedly suggested a trigger for a second independence poll could be if Scotland were dragged out of the EU against its wishes because of No votes elsewhere in the UK.

One of Mr Cameron's senior aides refused to be drawn into a "web of hypotheticals" and say whether or not the PM would facilitate a second independence poll should the UK vote to leave but Scotland vote to stay.

The Tory leader's decision to allow ministers to speak out mirrors a policy adopted by Labour premier Harold Wilson in 1975 and avoids the potential for a series of damaging resignations by senior UK Government colleagues.

However, it throws a spotlight on senior Conservatives such as Home Secretary Theresa May, Work and Pensions Secretary Iain Duncan Smith, Leader of the Commons Chris Grayling, and London Mayor Boris Johnson, who will all face increased pressure to declare their hands.

While the Vote Leave campaign welcomed Mr Cameron’s announcement, it was attacked by his political opponents.

Former Home Secretary Alan Johnson, who heads the Labour In for Britain campaign, described it as a “very dangerous and precarious step” not to have a united team.

Tim Farron, the Liberal Democrat leader, accused the PM of “putting his own internal party strife above what's best for Britain”.

Humza Yousaf, the SNP Government’s Europe Minister, said the decision to have a free vote among UK Government ministers was “deeply worrying and could potentially move the UK closer to the EU exit”.

During a Commons statement on the December European Council, which discussed Mr Cameron’s proposals for renegotiating Britain’s EU membership, he explained there would be a “clear Government position” on whether to leave or stay.

Once a deal was done, which the PM hopes will happen in February, “it will be open to individual ministers to take a different personal position while remaining part of the Government”.

Mr Cameron, who visits Bavaria today before travelling to Hungary to promote his EU reform programme, added: "Ultimately, it will be for the British people to decide this country's future by voting In or Out of a reformed European Union in the referendum that only we promised and that only a Conservative-majority Government was able to deliver."

The senior aide to the PM confirmed how a number of ministers had approached Mr Cameron towards the end of last year to raise the issue of collective responsibility; lifting it for the campaign period was “a mature approach to a very big decision,” he insisted.

Asked if a minister who opposed the Government line could lead the Brexit campaign, the aide replied: “They will be able to have their personal positions. We are not then imposing additional restrictions on what they can and can’t do.”

This leaves open the intriguing possibility that a senior anti-EU Cabinet minister could head the campaign for Britain to leave the EU.