After 24 hours of formalities that will end on Wednesday afternoon with the Queen inviting the new leader of the Tory party to form a government, the transfer of power from the 76th prime minister of the United Kingdom to the 77th will be a clinical and fairly brutal business.

“We have been told we will have to be out of the building – having handed in our phones – and have left through the Cabinet Office within half an hour of proceedings ending at the palace,” said a member of Theresa May’s Downing Street staff. “From that moment, we are out of a job.”

The only people left in No 10 during this short transitional period will be the permanent civil servants. The political appointees, including special advisers, will be out on their ears. With nowhere else to go, a group of May’s staff are planning a final get-together in a nearby pub on Wednesday afternoon, having cleared their desks. “There is not much messing around on these occasions,” said another staff member.

The process will begin on Tuesday at the QEII Centre in Westminster, when Dame Cheryl Gillan will announce the winner of the leadership contest on behalf of the 1922 committee of Tory MPs. The successful candidate, almost certain to be Boris Johnson, will, at that stage, merely become leader of his party. Another day will pass before he enters Downing Street.

May’s last parliamentary act as prime minister will happen on Wednesday when she takes prime minister’s questions before returning to Downing Street, where she will make a short valedictory speech, thanking her staff and bringing the curtain down on her troubled three-year premiership. She will then be driven to Buckingham Palace, where she will tender her resignation to the Queen, and advise the monarch that her successor as Tory leader should be invited to form a government.

Assuming there are no hitches, the Queen will then summon the new Conservative leader to the palace and ask him to form a new government. A prime ministerial limousine will wait at the palace to take him back down the Mall and Whitehall and into No 10. It is expected that he will make a short address outside before the job begins in earnest.

All new prime ministers like to give the impression that there is no time to waste. This time, with tensions rising in the Middle East after the seizure by Iran of a British flagged tanker in the strait of Hormuz on Friday, and with a Brexit deadline of 31 October looming, there genuinely is not a moment to spare. One former government minister with knowledge of the Middle East said yesterday: “If it is not resolved, the Iran issue will be top of the new PM’s pile. He will have to be straight on the case. What happens, for instance, if Trump wants to take action, rings him and says, ‘Are you with us?’ That is a big call.”

Johnson has already made it known to staff and civil servants that he will be taking almost no holiday in August – which, in effect, is an instruction to others to do the same. The message will be “all hands to the pump”.

The Queen welcomes Theresa May at Buckingham Palace in 2016, inviting her to form a new government. Photograph: Dominic Lipinski/PA

The first task will be to appoint members of the cabinet and government. “I would expect the new PM to aim to get the main cabinet jobs announced by close of play Wednesday, and for the government to be finalised by the end of Friday,” said a serving minister.

If it is Johnson who takes over, he will probably pull one or two surprises to make the cabinet look like his own creation, perhaps appointing more women to top posts. That said, there are strong indications that he will keep Jeremy Hunt at the Foreign Office. The volatile situation in the Middle East and potential for that crisis to escalate, together with the need for diplomatic progress on Brexit very soon, mean carrying on with a Foreign Secretary who is already up with the many briefs will have great attractions. The most high-profile change is likely to see Sajid Javid, the home secretary, become chancellor, replacing the disaffected Philip Hammond.

There will plenty of tough calls for Johnson, if he wins. Among them will be what to do with Michael Gove. Some Johnson-backing ministers believe he will leave the man who stabbed him in the back in 2016 in charge at the Department for Environment, Food and Rural Affairs, which has a big role in Brexit. Others think Gove could be switched to the Home Office, where the huge brief, including immigration and crime, will keep him fully occupied.

Then there will be the difficult task of rewarding those who played big roles in his campaign, including Gavin Williamson, the former defence secretary, and former Tory party chairman Grant Shapps, who managed the list of MPs known to be loyal to Johnson.

As well as Brexit and diplomatic challenges in the Gulf, the new PM will face big domestic choices and policy decisions, all of which will be made more complex by deep economic uncertainty. The government’s three-year spending review has already been postponed beyond the autumn because no one knows what state the public finances and the economy will be in until Brexit is resolved, one way or another.

Nicky Morgan, Tory chair of the Treasury select committee, said uncertainty over Brexit was having a knock-on effect across government. “It is clear from the evidence heard by the committee and from last week’s OBR [Office for Budget Responsibility] report that the final shape of Brexit will have a huge, and potentially adverse, impact on the UK economy and how much the government can spend on other urgent priorities,” she said.

“The fact that the delay in our EU departure date has put back the spending review, which departments really need to happen this year, shows how not knowing the shape of Brexit means plans for government spending are becoming impossible to finalise.”

Protesters against Heathrow expansion outside the Royal Courts of Justice. Photograph: Kirsty O’Connor/PA

Some big decisions cannot be put off, however. One that will have to be made by the end of this year is over the future of the HS2 high-speed rail network, the first phase of which is due to be built between London and Birmingham. Johnson has not sounded hugely convinced by HS2, and campaigners against the project believe they now have a chance to stop it.

Yesterday the Financial Times reported that the HS2 budget had risen by around £30bn above the previous total of £56bn. Campaigners and lobbyists who oppose the project say the new PM will have a chance, and good reason, to cancel it.

One of them, Deanne DuKhan, said: “Recently lobbyists for HS2 have scrambled to convince a sceptical public that it’s too late to stop the project but no construction has started yet, only preparatory works. Notice to proceed, which has been delayed by over a year, will now not be given by the government until early next year.

“A new prime minister would have an opportunity to redirect that massive funding envelope and distribute the investment across the entire rail network. Some genuinely vital and transformational projects could go ahead if the billions for HS2 were reallocated, including system-wide upgrades and intra-city schemes.”

Johnson will also come under pressure over the expansion of Heathrow airport, a cause he has long opposed. Could the new PM really call a halt to two big transport infrastructure projects?

Johnson previously promised to lie down in front of the bulldozers to stop the expansion going ahead, before softening his line a little more recently and saying he would be monitoring the legal cases on air and noise pollution.

Rob Barnstone, coordinator of Stop Heathrow Expansion, believes Johnson could well cancel the plans. “Boris Johnson knows that Heathrow expansion cannot meet environmental targets, including on noise and air pollution. He has indicated that he will be following the legal and planning processes very carefully – so at the appropriate time the project can be cancelled. We don’t expect any gimmicks, but remain confident that Mr Johnson will stop this disastrous project, albeit at the correct time in the process.”

The new prime minister will walk triumphantly into No 10 on Wednesday. But no occupant of the post in recent times will have had so much to contend with in so little time, both at home and abroad. In that sense it is difficult to envisage any prime ministerial honeymoon period at all.