Brian Monteith is policy director of ThinkScotland and editor of He is a former Conservative and Unionist Member of the Scottish Parliament.

It has been fairly common thus far for the debate surrounding Scottish independence to focus mostly on the economic ramifications for Scotland and the rest of the UK. Then the tangible spin-out of any subsequent divergent approaches on, say, tax and immigration, leads to consideration of differences, good or bad, that might result in capital flight or border controls.

The consideration of any political impact of a vote in favour of independence is usually limited to the rather simplistic and often ignorant mantra of  “England will forever become Tory” when we know from past electoral evidence that it need not. What has not been given any particular airing (in public at least) are the immediate ramifications for politics as played out at Westminster.

I suggest that now, before the debate begins to become more detailed and no doubt more heated, is a good time to think about that – for it is important that the government, and the Conservative Party in particular, waken up to the mortal threat that it faces.

Put very simply, consideration has to be given to what would happen at Westminster if Scotland decides to leave the Union.

I know I am not alone in thinking that there is a very strong case that the Prime Minister, David Cameron, would have to resign from office. It might be possible for the Government to survive, but it is difficult to believe that after such an abject rejection and public humiliation of his government by a member nation of the United Kingdom that the Prime Minister could retain the confidence of the House of Commons.

I certainly think it beyond reason for anyone to expect that Ed Miliband as Leader of the Official Opposition should see it as a mild discomfort for the United Kingdom and that it should be business as usual. It would be his duty to at least test the confidence of the House towards the Prime Minister.

Do we not expect party leaders to resign nowadays when they lose elections?  If Scotland leaves the UK the country will have lost over five million of its people and £127 billion (8.4 per cent) of its GDP – putting it again behind France at sixth in the World economic league table.

The Prime Minister will have presided over the disintegration of the United Kingdom as we know it. He will have been in political command of our land but suffered a peacetime defeat at the hands of Alex Salmond never achieved by Napoleon, the Kaiser, Hitler or any Cold War henchmen over previous Prime Ministers. The rupture will be greater than that of Ireland’s departure in 1922 for, after initial controversy over the passing of the Acts of Union in 1707, Scotland has been a generally willing and mostly successful partner in the achievements of Great Britain. There will have been no civil war – but there will have been a political and democratic rejection by Scots of the Britain now run by David Cameron – surely any democratic responsibility should exact a political price?

Are we honestly to believe that no one at Westminster will be held accountable for this turn of events?

In days past, one could expect that in such circumstances of losing a significant strategic battle – such as the fall of Norway – a Prime Minister might lose the backing of his supporters and tender his resignation to the monarch. Margaret Thatcher was threatened by this very scenario when the Falklands were taken by the Argentines but the House rallied round. Can David Cameron be sure of Liberal Democrat support if Scotland were to reject the United Kingdom?

Might a Yes vote not be a propitious moment for the Liberal Democrats to signal they no longer wish to support the Prime Minister?  As Deputy Prime Minister Nick Clegg could be expected to take the reigns at Ten Downing Street while the Conservatives chose a new leader – or he might explore what options existed in finding new partners altogether. Such events have a tendency to move very quickly. The clamour of the media for a head to roll in what could become a constitutional crisis may be irresistible. It only takes some sober constitutional and legal advice to go against the Prime Minister and he may see his room to manoeuvre severely limited.

I do not write this with any relish or having any particular axe to grind.  As a student I campaigned for a No vote in 1979, while in 1997 I almost single-handedly managed the ‘No No’ campaign – there is no prospect that I would do anything other than vote No on 18 September or wish that campaign ill. But I do despair about the complacency I find amongst so many unionists about the coming referendum and the carelessness of those who think it possible to brief against the Better Together campaign and its leader Alistair Darling as if the referendum is just another election.

If the outcome is a Yes it cannot be put right in five years.

As I said, I am not the only person to think the unthinkable. In a paper published this week by Walbrook Economics, a London-based independent economics consultancy, the author Ewen Stewart states that, while in the event of a Yes vote there would be no requirement constitutionally for the government to fall, David Cameron “would have no option but to resign” as the terms of the referendum set in The Edinburgh Agreement “were decided on his watch, and for the nation to suffer such a seismic shift without ‘falling on his sword’ would be unthinkable in terms of realpolitik.”

Let me put it another way.  If David Cameron is able to renegotiate the terms of British membership of the European Union and then put that deal to the British people – and it is rejected in favour of us leaving the EU – does anyone seriously think he could remain in office?  The Prime Minister would have set out the European strategy, he would have made the deal, he would have campaigned for it and he would have been rejected.  He would have to resign.

In this coming referendum David Cameron is defending the Union after cutting a deal with Alex Salmond, he may not be the No campaign’s front man but he is Parliament’s figurehead making the case emotionally, economically and culturally. The SNP is campaigning in particular on the partisan theme of avoiding any more Tory governments in Scotland – even one presided over by the thoroughly modern David Cameron.  It will thus be a defeat suffered by David Cameron and felt personally by him.

The ramifications of this must be understood by the Conservative Party, the Parliamentary Party and Downing Street – if the outcome is not being taken seriously yet, then they all had better get serious now.