Brian Monteith is Communications Director of Global Britain and a former member of the Scottish Parliament.

Back in 2014, as part of a series of articles I wrote for ConservativeHome about the Scottish referendum, I argued that were Scotland to vote to leave the United Kingdom, David Cameron would have no option but to resign.

My reasoning was rational enough: he would have negotiated the Edinburgh Agreement that was advantageous to the nationalists; a Prime Minister losing a sizeable and wealthy chunk of the UK would lose all authority and credibility in the Commons, and both the opposition and coalition partners could be expected to exploit such a humiliating situation.

The response to my case was mixed. Some agreed it would be impossible for Cameron to continue, while defenders of his position argued that since he had no vote in the matter he could hardly be held responsible, that he was generally outside the campaign and that the decision of Scots should have no bearing on his premiership.

I thought these defences were weak.  The Edinburgh Agreement could hardly have been more damaging, for it not only gifted the nationalists the ‘Yes’ answer for the referendum ballot, but also excluded Scots living outside the country who were already on the electoral roll and could vote in general elections, as well as allowing the long campaign that suited the SNP. The question, the franchise and the timing were determined on his watch.

The Prime Minister obviously shared my analysis, since he began to play more of a role in the campaign, culminating in his famous last-minute “vow” with the other party leaders to confirm that the Scottish Parliament would have more powers. Indeed, it later came to light that he had conceded privately that he would have felt under great pressure to resign had Scotland voted to secede.

Fast-forward to the coming EU referendum – which starts with the opposing camps closer than they were in the Scottish referendum at this stage. A vote to stay in the European Union cannot be assured, and the Remain campaign has got off to a lacklustre start. The two Leave campaigns have a spring in their step and each bring different strengths in research, analysis, optimism and popular reach that the Remain camp appears to lack. When the Leave campaigns merge – as they will have to do – it will be a formidable combination. Add to this the unpredictable existential crises that have a habit of making the EU unpopular and it would be a fool that would put his house on choosing the winner.

If the nation votes to leave there can be no doubt that Cameron must fall on his sword. The case for his departure will be even stronger than it was with the Scottish referendum – and he will know this. As I wrote last year, “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.”

For the Conservative Party that might not be too important, since he has already said he will not seek to be Prime Minister after 2020. In such circumstances, the Government would likely remain in office, but the Party would be thrown into an unwanted early leadership tussle that would feed off divisions that would have by then have already become heightened.

The received wisdom is that George Osborne will in time become Cameron’s replacement, but this is without considering a vote for Brexit. While the lines of accountability for such a political humiliation would trace back to the Prime Minister, I believe that the public would consider responsibility to be shared with the Chancellor, too. Many Conservative backbenchers and ministers would also feel that way.

The chief negotiator is the Chancellor, it would be the terms that he personally negotiated on the Prime Minister’s behalf, hand-in-glove, that would have been rejected. Replacing the latter with this chief architect, who would have defended the renegotiation line-for-line, would not be tenable with the British public. An immediate replacement would be required, but a divided party facing a lengthy leadership contest could hardly turn to Osborne when he had played so large a role in Cameron’s downfall.

With the Prime Minister departed, and their joint proposals in tatters, how could the Chancellor be expected by either Conservative members or the wider public to then negotiate the UK’s Brexit terms with the EU? There is currently no Deputy Prime Minister, but even were this person to be Osborne the courtesy title could still not save him.

Only a senior minister who had displayed identifiable EU-sceptic sentiments, and had chosen to campaign for a ‘Leave’ vote – perhaps having to quit the Cabinet in order to do so – could claim the necessary moral authority. It is too early to speculate whether such a person might be Theresa May, Sajid Javid, Boris Johnson or someone yet to shine during the campaign, but it could not be the Chancellor of the Exchequer.

If Osborne truly wants to be Prime Minister he should be prepared to be honest with himself and say if the deal he is trying to negotiate does not serve the UK’s best interests it will be better to leave the EU than remain. The Prime Minister may not like it – but Osborne’s neck is on the line too.