Tory peer Lord Fraser of Carmylie will take part in a public debate in London next month backing the motion that "It's time to let Scotland go".

He has agreed to appear on the same side as writer Gerry Hassan and former Sun editor Kelvin MacKenzie, arguing the case for Scotland's departure from the United Kingdom.

He told The Herald last night he was not predicting the outcome of the 2014 referendum, nor saying he wanted to see independence happen, but expressing his view that the decline of Unionist sentiment north of the Border made it inevitable in the longer term.

He added: "Once upon a time you could travel from the Cromarty Firth to the Border, having coffee with me in Dundee and lunch with Malcolm Rifkind in Edinburgh and pass through Tory territory all the way.

"Nowadays if the Tories gained five MPs in the whole country there would be a roar of champagne corks going off, but the truth is there will be no recovery for the Tories this side of independence.

"I also sense a significant shift in England – thanks to First Minister Alex Salmond's skilful handling of events – of people saying, 'If you want it, off you go," and I think his talk of England losing a surly neighbour and gaining a good friend has been a very good line."

Lined up against the motion in the planned Spectator debate on June 27 are two Scots Tories representing English constituencies – Sir Malcolm Rifkind, MP for Kensington and Rory Stewart, MP for Penrith and the Border.