Exceptionally for a modern political leader, Theresa May does not employ a designated speechwriter. This singular fact about May’s Downing Street says many things about the way this prime minister works. The contrast with her recent predecessors is deliberate, and the explanation is obvious: May doesn’t employ a speechwriter because she doesn’t make many speeches.

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The woman who David Cameron’s aides called “Submarine May” prefers doing politics in private. When it comes to speeches, her clear preference is to confine herself to those that go with the job. She gives a speech to her party conference, another to the lord mayor’s banquet in the City, and those required at formal functions and on foreign visits. That’s about it.

Most of May’s speeches and statements to MPs are written by officials. The political parts of her speeches are mainly entrusted to Chris Wilkins, the No 10 strategy director – back in 2002 he was an author of May’s “nasty party” speech – and to Nick Timothy, her joint chief of staff. Both advisers share May’s disdain for what they see as the frantic, headline-chasing initiatives of Cameron’s No 10.

But the significance of May’s reluctance to speak in public has not been widely grasped. Unlike Tony Blair, Gordon Brown and Cameron, May does not aim to use what Americans call the bully pulpit to shape public debate. It is one of the many changes of tone and practice that this unshowy politician has brought to the prime ministership. Her regime is marked by a lack of display that in some ways harks back to a more deferential political culture. Whether the approach works in the madhouse of the digital era has yet to be proved.

All this underlines why the speech May will make about Brexit policy this month will be so unusual and important. May would not be making the speech unless she had been forced to. But ever since the House of Commons voted at the start of December for Keir Starmer’s Labour motion demanding a published government plan for exit negotiations with the EU, she has had no choice. Now the speech has been turbo-charged with fresh significance, after Ivan Rogers resigned as the UK’s ambassador to the European Union this week.

The exact date of the speech has not yet been set, but it is seen in No 10 as the only one on the subject that May will make before triggering article 50 of the Lisbon treaty in March. One delicate question is whether May will make the speech before or after the supreme court hands out its long-awaited ruling on the government’s appeal over parliament’s role in that process. But the Rogers resignation, combined with the Commons vote and the supreme court ruling, means that May has to regain the Brexit initiative on multiple fronts.

Facebook Twitter Pinterest The resignation of Sir Ivan Rogers as the UK’s ambassador to the EU ‘means that May has to regain the Brexit initiative’. Photograph: Thierry Roge/EPA

It is one thing to report that May intends the speech to regain the initiative. It is another thing for this to happen. The political realities, after all, have not changed. These realities remain, first, that control over EU migration is not compatible with membership of the single market; second, that there is no reliable Tory majority at Westminster either for restricting freedom of movement or for staying in the single market; and third, that Brexit is a multilateral negotiation involving 27 EU member states and the European institutions. These things all remain true. The idea that May can solve all these fiendishly difficult problems with a single speech is absurd.

It is also very unlikely that she will say something different in early 2017 to what she said in 2016. To do so would open her to the charge of flip-flopping, and would anyway be out of character. According to those familiar with her thinking, it is therefore best to return to her last substantive speech on Brexit as a guide. To do so is to see the immensity of the moment towards which the UK is now hurtling.

At the start of the Tory party conference, using the same phrase that Rogers unleashed this week in his resignation memo to staff, the prime minister derided a choice between hard and soft Brexit as “muddled thinking”. There would be no trade-off between controlling immigration and trading with Europe, she said. Britain would leave the EU, would take control of immigration, and would remove itself from the jurisdiction of the European court of justice.

That speech was widely interpreted as a hard Brexit statement, not least in the rest of the EU, because that is exactly what it was. This month’s speech will not, perhaps cannot, budge from those objectives. That’s because, more than anything else, May has to be able to turn round to British voters and say: “Look. You voted to leave the EU – and we have left it.”

The litmus test for that approach remains migration controls. Doubtless May would like to sugar the pill for economic pessimists, as she did in the conference speech, by saying that trade-offs with the EU will be part of the outcome, and that there will be areas of compromise and cooperation, including law enforcement, security and some aspects of trade. Yet a lot of this feels like wishful, if not muddled, thinking – which was presumably the breaking point for Rogers.

In the end, the core political reality of the Brexit negotiation remains the one inflicted by the referendum. That result, for May, means leaving the EU on terms that the voters will accept. The prime minister, and many others, believe this can only mean UK controls on EU migration. The chancellor, Philip Hammond – and maybe even May herself – hope the door can remain open for certain privileged categories. But the bottom line is to stop, and to be seen to stop, migration flows.

May may be reluctant to say it in a speech, but the facts point in one direction. She wishes not just to deliver a Britain that is out of the EU, but a Britain that puts border controls above today’s mix of global macro-prosperity and widening inequality.

The course she is on means putting the interests of leave voters above the economic interests that have always shaped the Tory party. If she has to say to voters that the price of controlling migration may be economic hardship, she will have to say it. From one perspective this is utterly muddled and illogical. But from another, it seems very clear indeed.