More than two years have elapsed since British Prime Minister David Cameron gave the green light to the Scottish referendum. At the time, it did not seem that big a deal, either to the prime minister or to anyone else. It was generally assumed that only a minority of Scots would vote to end the Union. There is some evidence that Alex Salmond of the Scottish National Party (SNP) thought so. He wanted to put a third option — greater powers falling short of independence — before the voters.

However, Cameron thought that he could put Salmond on the spot with a simple ‘Yes’/’No’ referendum. This has proved a giant miscalculation. Last weekend, panic-stricken Westminster politicians offered the SNP leader his “devo-max” anyway. Yet, it still looks possible that Scottish voters will support full independence. Such an outcome will create Britain’s greatest constitutional crisis in more than 300 years, dwarfing the famous clash between the House of Lords and the Asquith government 100 years ago, or the Great Reform Act of 1832.

Let us consider the immediate consequences. There will be agitation to extend the term of the current parliament to March 24, 2016, the date at which Scotland will become fully independent. This manoeuvre has been floated in order to solve the problem of Scottish MPs returning to parliament after next year’s general election, only to find that their seats at Westminster will cease to exist the following year. But the general election is unlikely to be delayed — British democracy does not allow governments to extend beyond their natural terms, except during wartime.

Another possibility is more plausible: An early general election. Cameron is adamant that he will stay on as Prime Minister even if the worst happens next Thursday — an erroneous opinion, which has been endorsed by Ed Miliband. Cameron may well be wise to say what he does, because he does not want to place his personal fate at the heart of the Scottish plebiscite.

However, 30 years ago, Lord Carrington resigned as foreign secretary after the Falkland Islands were captured by Argentina. Lord Carrington quit his office, even though he was not the slightest bit responsible; and the islands were recaptured a few weeks later. If everything goes wrong this week, Cameron will have mislaid Scotland, not a group of largely uninhabited islands in the South Atlantic.

I am certain the prime minister, an honourable man, will resign at once. If he does not do so, very little time will elapse before he will face a motion of no confidence on the floor of the House. The Conservative Party may then be plunged into a leadership contest. To avoid this kind of chaos, some Tory MPs are now talking of a caretaker — almost certainly William Hague — taking over as Tory leader (and prime minister) for the last few months before next year’s election. Ed Miliband may also find survival difficult, because the Labour Party must surely bear the bulk of the blame for the loss of Scotland, where the party has traditionally been so strong. And amid all the chaos and recrimination, politicians will be obliged to make a series of administrative decisions of exceptional complexity and sensitivity.

Malevolent new English identity

National partitions — think of Ireland in 1922, India in 1947, Southern Sudan more recently or the birth-pangs of a Kurdish state now — are always bitter affairs. This is because two sides are arguing over the spoils. Who owns what? To whom do the armed forces give allegiance? Who inherits the debt? Who keeps the currency? There is already evidence of sectarian hatred, both north and south (where there are signs of a malevolent new English identity) of the border. All of these issues will have to be dealt with by a set of politicians reeling with shock at the collapse of many of the landmarks that have given meaning to their lives. The BBC will have to find another set of initials. Former premier John Major noted recently, during a particularly depressing appearance on the Today programme, that Britain’s seat on the UN Security Council will go and hinted that Wales could be next to secede.

The Queen — the head of the Union — has decided to remain above all this. It is true that she has always understood the importance of remaining neutral and above politics. This, indeed, has been one of the secrets of her magnificent reign. But this vote is not about party politics. It is about the survival of the British state. The Queen’s decision not to make her views known shows that her advisers (most notably her Private Secretary, Sir Christopher Geidt, and the prime minister) have failed to understand the constitutional role of the contemporary British monarchy.

Professor Philip Murphy brilliantly demonstrated in his recent work Monarchy and the End of Empire that the Queen has always had a twin identity — British Head of State as well as monarch of 15 other realms, ranging from Papua New Guinea to the Bahamas. Her British identity has always come first. For instance, whenever the Queen has attended the Commonwealth heads-of-government meeting, she has been obliged to take the advice of her British prime minister — even if it infuriates her other prime ministers, as it often did during the Commonwealth rows over South African sanctions. Salmond — and for all I know the Queen — may think that everything will continue as before, but that is simply not true.

If Scotland opts for independence, the Queen will certainly remain Head of State north of the border, but her relationship with Salmond will closely resemble her relationship with Peter O’Neill and Perry Christie, Prime Ministers of Papua New Guinea and the Bahamas, respectively. This will be very difficult, because it is hard to see how the Westminster government and the new Scottish government can avoid coming into conflict. Professor Murphy’s book contains a fascinating quote from a minute by a British official at the time of Nigerian independence, noting that “if there were any possibility of Nigerian policy developing on lines that she would dislike, there is a good deal to be said for the Queen not being Head of State”.

Let us imagine that the First Minister of Scotland, Salmond, placed words in the mouth of the Queen that were critical of the government in England. She (or her successor) would find herself in an impossible position. For that matter, it is very unlikely that her English subjects will take kindly to the Queen going on holiday in a foreign country. At present, she makes visits to her dominions abroad, but has no permanent residence in Australia or Canada, as she does at Balmoral. In the fullness of time, this capacious and historic residence looks doomed to become the summer retreat of Scotland’s First Minister.

And let us remember that the Queen has acknowledged the importance of the Union before. She remarked that her 1977 Silver Jubilee was “a time to remind ourselves of the benefits that union has conferred, at home and in our international dealings, on the inhabitants of all parts of the United Kingdom”.

That remark remains as true today as it did then. The Queen has reigned marvellously for more than 60 years and it is most unfair that she has been faced, at the age 88, with the most important decision of her reign. But if everything goes wrong this week, how she will regret not having done more to fight for Britain.

— The Telegraph Group Limited, London, 2014