Liam Fox’s intervention in the EU referendum debate has unleashed a great deal of speculation about how the Conservative leadership should handle the party in the run-up to the EU referendum. In particular, how should the prime minister handle the question of ministerial collective responsibility? The answer? Ministers should be free to speak. This is not a new idea. It’s what happened in the 1975 referendum.

“Ministerial collective responsibility” is a fine convention of our constitution, enshrined in precedent and, in more recent decades, written into the ministerial code of conduct. It is the principle that if you are a minister, you do not criticise the policies of your fellow ministers whenever you like. It is agreement to disagree in private and to accept in public there is only one government view. You work together, you compromise. Quite a lot of the time, to keep things orderly and to avoid distraction, you button your lip. You even say things you do not agree with. It just goes with the territory.

Every now and then, however, an issue arrives that is so fundamental and so existential for party and country that for your own self-respect you have to speak out. Then the prime minister has a choice: either to suspend ministerial collective responsibility, or to start sacking ministers who cannot in all conscience toe the line.

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The EU referendum is one such issue. This referendum dwarfs the significance of a single general election, which only decides the direction of the nation for five years. This is a once-in-a-generation choice for all our futures. Unless the political class is to lose yet more public respect, all politicians should be free – and encouraged – to air their differences.

In such a situation, the issue itself – and respect for the choice to be made by the voters – are the highest consideration. Before 2010, David Cameron was fond of echoing Winston Churchill and advising his candidates “country before party”. The most important consideration is the question itself. In the interests of a fair referendum, in which there is the maximum of debate, the prime minister should invite his own ministers to say what they think, without requiring them to resign. To hear Michael Heseltine, of all people, weighing in against this freedom should be taken with a pinch of salt. What is the point of a referendum if debate is stifled by muffling key voices?

Moreover, there are strong arguments for ministers demanding to be let free to speak now. The whips are arguing that it is disloyal to declare for “leave” now, before the negotiations are concluded, but the prime minister has made clear he is already for “remain”, himself prejudging the outcome.

Having opposed the Lisbon treaty, all Conservatives can see that the renegotiation is now just a front for accepting the Lisbon treaty in full, as it is. There is no way the prime minister now intends to deliver on the Bloomberg principle that. “It is national parliaments, which are, and will remain, the true source of real democratic legitimacy and accountability in the EU.” Even if he gets everything he is currently asking for, the rest of the EU will, as now, still be able to outvote the UK parliament on everything.

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He has utterly abandoned the 2010 manifesto’s “three guarantees”, as set out in his 2009 speech after the ratification of Lisbon. This called for “the return of Britain’s opt-out from social and employment legislation”, “a complete opt-out from the [EU] charter of fundamental rights”, and “an arrangement … limiting the European court of justice’s jurisdiction over criminal law to its pre-Lisbon level”.

Just before the 2015 election, he told the House of Commons his renegotiation was “the opportunity to reform the EU and fundamentally change Britain’s relationship with it”. Not a hope. Former prime minister John Major’s resort to “subsidiarity” on Sunday underlines how today’s policy is now repeating, 25 years on, the same “game, set and match” style of triumph of hope over experience. The European court of justice always gets the last, centralising word.

Cameron should still also consider the best interests of his party. Nobody should doubt that the Conservatives are split on the EU question. Many of us warned the in-out question was divisive. The only way to bring Conservatives together again after the referendum is to be able to say to everyone, “You had your say.” In particular, George Osborne must put aside any idea that he can dragoon ministers and MPs into supporting a position most of their constituency members and voters will not.