When the next prime minister walks into No 10 this week he will have only one real subject on his desk – Brexit – but a host of decisions to take.

First, he needs to pick his cabinet. The balance of Leavers and Remainers will send a signal to voters, parliament and the EU about his intentions. For Brexit, two choices matter most – those of chancellor, and head of the Brexit department.

Indeed, one decision is whether he should still have a Brexit department. There were good arguments that the Department for Exiting the European Union should never have been created – that Brexit should have been run from No 10 and the Cabinet Office. But given that it does exist, and time is painfully short before the biggest test the new prime minister will take, it would be reckless to get rid of it.

Boris Johnson. Photograph: Dan Kitwood/Getty Images

The choice of who runs it will help determine the tone of talks with the EU. But the choice of chancellor matters even more. Both Boris Johnson and Jeremy Hunt have said that they are prepared to press ahead with no deal. They will need a chancellor who can argue the case for the government’s course in the face of forecasts – the latest, last week, from the Office for Budget Responsibility – predicting that no deal will trigger a recession.

Next, he needs to decide who will replace Olly Robbins, the civil servant who led EU negotiations under Theresa May. Given that Johnson and Hunt say they reject her deal, they will not want to keep him. They might instead want a political appointment. But they will have to be clear how much freedom the person has – and how they will replace the detailed knowledge that Robbins had of why clauses in that deal were worded as they were.

Given that the European team has changed too, some would say a new team on the UK side is worth the loss of recent knowledge. But the new prime minister will have to be clear to himself and his team about where he is prepared to compromise. He also needs to be clear about where there is realistic hope of an EU concession – say, within the non-binding political declaration attached to the withdrawal agreement, or on another extension. EU officials may be prepared for low-key talks over the summer, but have discouraged hopes of much revision.

Jeremy Hunt. Photograph: Peter Nicholls/Reuters

However, Johnson has repeatedly ruled out a further extension, and last week ruled out other areas of potential concession, such as attaching a time limit to the Irish backstop. If he is prime minister, he will need a plan from the start – meaning this week – about how to approach negotiations if they are to look like a serious attempt to find a deal.

The prime minister will also need to ramp up no-deal preparations in Whitehall. If the UK does exit without a deal, showing that preparation had been done will be central to a prime minister’s ability to persuade the country that it was justified. He will have to communicate to businesses what he expects them to do. Many smaller businesses have not yet prepared for no deal, relying on the government to try to avoid it.

Finally – but still this week – the prime minister will need a plan for getting support from parliament and a decision about whether to try to sidestep it in the event of no deal. As the manoeuvres by Dominic Grieve, Keir Starmer and other fierce opponents of no deal show, it is very hard for parliament to find a mechanism for blocking it. But the uproar if a prime minister tried to shut the House of Commons out of the most significant question of recent times would threaten to bring his time in No 10 to a rapid end.

Even if three months might seem a long time in politics, a prime minister cannot hope to chart a course to 31 October without taking many steps by the end of the coming week.

Bronwen Maddox is director of the Institute for Government