Imagine if the Scottish independence referendum campaign in 2014 had been conducted without collective cabinet responsibility; if figures such as John Swinney or Alex Neil (say) had been allowed to campaign for Scotland to remain in the UK while Alex Salmond and Nicola Sturgeon argued to leave.

It would never have happened, of course, because all the senior members of the SNP were united in voting Yes. But make no mistake; David Cameron's decision to abandon collective cabinet responsibility is just as divisive, potentially, as that scenario would have been. For many Tories, UK independence from Europe is just as “existential” a question as Scottish independence is for the SNP

The Prime Minister has made clear that members of his Conservative cabinet will be free to campaign to leave the EU irrespective of the outcome of his negotiations on a new relationship with Europe, due to be completed in February. It represents a significant U-turn on his previous position, which was that his Government was “a team” and would have to stick to a coherent collective view on the most important issue regarding the UK's relations with Europe since 1975.

It is, of course, a recognition of political reality in the Tory party. Cabinet figures like Chris Grayling had made clear they would not be whipped into supporting a deal they believed was not in the UK's interest. There's been speculation that Home Secretary Theresa May wants to be a leader of the out movement and that the Mayor of London, Boris Johnson, might also see leading Vote Leave as the next stage in his career.

Does this matter? Some commentators have pointed to the previous UK referendum on the-then European Economic Community in 1975 and said that the Labour cabinet of Harold Wilson was similarly divided. Figures like Tony Benn campaigned to leave the EEC. But the divisions in the Labour party over Europe were never on the same scale as those in the Conservative Party today.

For Labour in 1975, Europe was not an existential question about the future of the UK as an independent country. No one believed that the EEC was a federal superstate, as many Conservatives do today, or that Westminster was becoming subordinate to the institutions of the EU, which didn't come into existence until the Maastricht Treaty was ratified in 1993.

Maastricht split the Tories down the middle and led to such profound division that the-then Tory Prime Minister, John Major, resorted to describing his eurosceptic cabinet critics as “bastards”. The division has never truly been healed.

The Conservative Party's hostility to the EU is arguably greater today even than in the early 1990s. Mr Cameron's renegotiation on Britain's place in the EU has persuaded few of his critics that the country's sovereignty is secure. And it is hard to see how anything can be cobbled together in Europe before February that would win them over.

The Prime Minister's objectives were always pretty threadbare: removing reference in treaties to “ever-closer union”; reducing red tape; ending the rights of EU migrants to claim benefits for four years. He has failed so far on the last demand. It seems likely that there will only be special dispensation for Britain to deprive migrants of benefits for two years on a temporary basis.

In other respects, of course, the EU has evolved in a way the UK Government wished. The Schengen zone of passport-free movement seems to be on its way out, with even Denmark putting up border posts against migration from liberal Sweden. The refugee crisis has led to a restoration of national sovereignty among the 28 member states as they try to secure their borders.

But this only confirms how modest the Prime Minister's original negotiating stance really was. Many of his backbenchers, and prominent Tory “outers” like Liam Fox the former defence minister, say Britain should waste no time in ensuring that the EU is never again permitted to threaten Westminster sovereignty. He wants Britain to leave and become part of a European Economic Area (EEA) with countries like Norway and Switzerland.

Perhaps the most important question is over the position of the UK civil service. Since the Prime Minister accepts that leaving the EU is clearly a possibility, will officials in the Foreign Office be allowed to give fair and balanced assessments of the prospects for Britain either way?

Indeed, should there not be some legal advice sought on how disengagement from Europe could be conducted and how Britain would enter the so-called European Economic Area? Indeed, what is the EEA?

We all recall the rows over Alex Salmond's claimed legal advice on the situation for Scotland in Europe if it left the UK. We are in a similar situation now. How can UK voters make an informed decision if they don't know what the mechanism will be for withdrawal from the EU?

And how will voters in Scotland know where they stand if they vote to remain within the EU while the UK votes to leave? Mr Cameron has made clear that, as far as he is concerned, this is not an issue since Scotland voted to remain in the UK in September 2014. If it's one out, it's all out as far as Britain is concerned.

But the SNP will still argue that the legal situation needs to be clarified. At the very least, there needs to be a clear withdrawal prospectus on issues like fishing quotas, energy policy, infrastructure funds and so on.

How long would it take for Britain to leave the EU? What penalties might be imposed? Would Scotland still be part of the single European market? Would Scotland and the UK still have to make payments to the EU even after withdrawal, as countries like Norway do, and would Scotland still be subject to the complex sets of laws that apply to the single market?

Would Scotland still send MEPs to the European Parliament? Presumably not since the EU parliament is an institution of the EU. So how would Scotland be represented in the EEA? Who exactly would conduct the negotiations on joining the putative EEA and would Scotland have its own position?

These are huge questions. In 2013, the Scottish Government delivered a 670-page White Paper on Scottish independence. Where now are the white papers on UK independence, in or out of Europe? We could be only six months away from a referendum on this most complex issue.

However, if the UK Government did provide alternative prospectuses for Britain's relations with Europe, and took seriously the case for withdrawal, that could itself have an impact on the result of the referendum. It might even persuade some Scots, who are not particularly enthusiastic about the EU, to rethink their position on staying in.

Could that even revive the long dormant strand of anti-Europeanism in the SNP? It's not that long since many Nationalists in Scotland were as hostile to Europe as nationalists are today in England. Times don't get much more interesting than this.