The prime minister must have wished to be anywhere but Balmoral explaining to Her Majesty that a poll was about to be published showing he was on the verge of overseeing the break-up of her most precious possession, the 300-year-old union. But saying sorry is not always enough to assuage one's sense of guilt.
Inevitably, the speculation has been that Cameron will have told the Queen – in one of the few British political meetings that remains truly private – that he will offer her his resignation if the worst happens next week. However, in public Cameron has insisted repeatedly he will not resign if he turns out to be the prime minister who presided over the break-up of the three-centuries old union. He told the BBC last week: "What's at stake here is not this prime minister or that prime minister, or this party leader or that party leader. What's at stake is the future of Scotland.
"It's very important for the people of Scotland to understand that this is about the future of Scotland."
But this commitment is only to ensure the Scottish people have a single issue in mind when they mark their ballot paper next week, and that is the question on the ballot paper, separation or not.
Cameron has also said he will be heartbroken if Scotland votes for separation, and he is a man of sincere emotion. Cameron himself in his "quiet patriots" speech on the union delivered in February at the Olympics Velodrome explained: "Some people have even advised me to stay out of this issue – and not to get too sentimental about the UK. But frankly, I care far too much to stay out of it. This is personal. This is our home – and I could not bear to see that home torn apart. Centuries of history hang in the balance. A question mark hangs over the future of our United Kingdom."
If the UK is broken up on his watch, he may sense his political epitaph is already written in stone. He will be the Lord North of the 21st century and continuing in office is pointless. But would a human sacrifice be required?
Many Tory MPs say it would be his duty to stay to negotiate the terms of the separation, dampen the crisis in the markets, rather than plunge his own party into the chaos of an unpredictable leadership election eight months before the 2015 election. A rudderless government is hardly going to reassure investors. They also point to polls showing most voters do not think he would have to quit, although that public opinion might change faced by the reality of a yes vote. There is, of course, no chivalrous code to which ministers can refer setting out when they resign on a matter of personal policy failure, as opposed to misconduct, or principle. Such resignations are a matter of personal judgment.
Although Estelle Morris quit as education secretary sensing she was not up to the task, Lord Carrington was the last cabinet minister to resign, along with his team, as a matter of honour saying the Foreign Office he led had failed to foresee the Argentine invasion of the Falklands. Chamberlain did not resign over his misjudgment of Hitler's intentions. Eden did not resign in 1957 due to Suez, but was instead ordered to do so by his doctors due to his failing health.
Yet there will be those who will say Cameron is not just the captain on the ship when the vessel hit the rocks, they argue he made a succession of mistakes that allowed the currents to create this shipwreck, and so is personally culpable.
The charge sheet will be that he was outmanoeuvred by Alex Salmond. Faced by Salmond unexpectedly winning a majority in the Scottish parliament in 2011, Cameron felt he had to respect that result and give the Scots a chance to vote on their future. He was responsible for the referendum's date and terms: the lack of a turnout threshold, 16-year-olds being allowed to vote – and Scots living outside Scotland not getting a vote.
The prime minister's major mistake, it is argued, was to take a calculated gamble. In the Edinburgh Agreement in 2012, Cameron vetoed any attempt to include so-called devo max on the ballot paper, making a firm commitment not to discuss any further constitutional change ahead of the referendum, because that would only cloud the issue of separation. It was all or nothing.
Once he set the terms of the debate, Cameron largely vacated the field to the Better Together campaign, refusing to debate with Salmond directly. Through no fault of Cameron's, the Conservative party has since the mid-80s lost almost all support in Scotland, largely due to the unpopularity of Lady Thatcher. Indeed as opposition leader in 2006, he apologised to Scotland saying his party had committed "a series of blunders" in Scotland in the 70s and 80s of which the poll tax was "the most egregious".
Although he was largely left on the touchline, he was sent open warnings chiefly by his predecessor, Gordon Brown, that countries can be lost by mistake. In a speech on 9 June Brown warned: "If people come up to Scotland and say, 'Britain says no to Scotland having a share in the currency, Britain says you are not going to have defence jobs', or 'Britain says you are going to go bankrupt', or, as last week, 'Britain says if you vote for independence you will lose your fish and chips and your pies and your holidays in Torremolinos' in a patronising way … It is obvious the issue will become Scotland v Britain and people will say the debate is 'Do you stand up for Scotland or are you for Britain?'."
Yet, having kept devolution off the ballot paper, the Scottish parties over-rode Cameron earlier this year, offering further packages of devolution beyond those set out in 2012 Scotland Act. Now faced by defeat, the Scottish unionist parties are suddenly arguing that a no vote is actually a yes vote for further powers for Scotland.
This leaves the UK as a whole in a constitutional mess, raising questions about why other parts of the UK are not being consulted on this further increase in Scottish power. It is an issue that is already causing a political backlash among English Tory MPs as they look at the scale of the chaos. Cameron can reasonably argue he was not responsible for the Better Together campaign and the true problem lies with Labour voters defecting to yes. If Cameron has to go, then Miliband would also need to consider his position on this thinking.
But the 1922 committee of Tory backbenchers – not over-populated by Cameron loyalists – has started to make discreet soundings on how to respond to a yes vote. Their exercise is only intended to be procedural at this stage, but Cameron will know his future may not rest in his own hands.
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