Becoming prime minister is a shock. Not so much the moment of being elected – any sensible democratic politician knows that opinion polls can be wrong and gets ready for all eventualities.

No incoming prime minister can ever be fully prepared for the demands of the job, placed on them from day one, when – among many other demands – they are taken aside to be briefed about their role in a nuclear war.

The process is particularly challenging in the UK because the change is so quick. There are no weeks of transition as in most other countries. Nobody else does it like us.

As Tony Blair remarked to Alastair Campbell: “Imagine preparing for a new job by working flat out travelling the country for six weeks and then go a few nights without sleep.”

If this general election goes to usual form either Prime Minister Keir Starmer or Prime Minister Rishi Sunak will be installed in 10 Downing Street by lunchtime the day after the vote.

Neither of them will have had any sleep the night before, waiting for the declarations in their own constituencies into the small hours and then dealing with the fallout from the results elsewhere.

If he has stayed in touch with reality, Sunak would certainly be flabbergasted by victory, given the general expectation that he would lose.

Re-election to the job of premier should hold no other surprises beyond trying to step around the elephant traps he has carefully dug for the next prime minister, assuming that it would not be him.

First-timer Starmer would face the challenge of taking on a job and lifestyle which only 56 people have ever experienced before.

Becoming prime minister at an election with a change of governing party is even rarer. There have been 13 general elections in the last 50 years but only three handovers of power between Labour and the Conservatives.

Being a senior minister is not an adequate preparation for Number 10.

Gordon Brown was a hugely powerful chancellor of the exchequer for a decade who regarded himself as a co-prime minister, yet a few months after he took over the top job, a senior Brownite ruefully confessed to me: “We thought it was going to be like the Treasury only bigger. It isn’t. That was handling just one thing. As prime minister everything comes at you from all directions.”

Along with taking tea with Margaret Thatcher, the former finance minister also spent his honeymoon period dealing with terror attacks in London and Glasgow and unexpected summer flooding across England.

Unlike Harold Wilson, James Callaghan, Edward Heath, Thatcher, John Major, Theresa May, Boris Johnson, Liz Truss and Sunak, Starmer has never served in a government as a minister.

He shares this lack of experience with David Cameron and Blair, who had been in parliament for 14 years when he became prime minister and a shadow minister for 10.

Cameron had been an MP for nine years when elected prime minister, as would be the case for Starmer, who only became an MP in 2015.

Cameron already knew his way around government having worked as an aide in Conservative headquarters and for senior ministers.

Starmer likes to boast that he had a successful career as a lawyer before entering parliament. He believes that running the “big organisation” of the Crown Prosecution Service should be good preparation for the premiership.

Starmer also says he knows how to cope with a change of style because he switched from poacher as a defence barrister to gamekeeper as director of public prosecutions.

A prime minister who comes at a general election usually has to switch in a moment from all-out campaigning to managing a party, a government and a country.

Except for Cameron whose preparations benefitted from a hung parliament and five days of negotiations to set up the coalition with the Liberal Democrats.

At least general election-elected prime ministers start with a clean sheet of policies and with plenty of jobs to hand out.

Blair admits “the disadvantage of a new government is lack of experience in governing” but he claims “it is also an advantage… we thought the unthinkable. We did the undoable.” His early gambits included shifting PMQs to one half-hour session a week and granting independence to the Bank of England.

This baptism of fire and new beginning perhaps explains why Thatcher, Blair and Cameron are the significant national leaders of recent years, who won re-election, rather than those who took over power by default of internal party machinations.

Sunak can never be a member but Starmer would have a chance to join this distinguished club, although he is circumscribed by the state of the economy and by the things which Tory campaigning has forced him to rule out.

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Incoming prime ministers depend heavily on the staff around them. Someone has to make preparations for government which could be only days away. A leader is occupied fighting an election campaign and must not be seen to be presumptuous or complacent about victory. Officials and even family members are slapped down if they slip saying “when” not “if” about winning.

Blair is credited with pulling off one of the most successful transitions thanks in large part to his team. His chief of staff Jonathan Powell and advisor David Miliband secretly drew up a plan in advance for the government’s first 100 days. Peter Mandelson and Alastair Campbell practically invented the arts of political spinning and media handling in this country. Anji Hunter and Sally Morgan had the delicate task of reaching out to the party and the outside world and managing the appointment of ministers and government advisors. Mistakes are sometimes made when handing out jobs, names may be mixed up, post-it notes dropped or mobile phones mislaid.

Starmer’s key decision in preparation for government was the controversial hiring of a widely respected senior civil servant as his chief of staff.

He has made it clear that Sue Gray will take over as top dog from campaign director Morgan McSweeney from the moment of victory. Gray knows everyone in Whitehall after decades of working there. She is expected to oversee Olly Robbins replacing Simon Case as cabinet secretary. She will also have a decisive voice over the appointment of advisors and ministers.

In the past, shadow ministers have had more than a year for “access talks” about their plans with officials in relevant government departments. Sunak withheld permission for these to start until early this year and has now called a snap election, meaning Labour has had barely six months to prepare.

Some of those involved in getting MPs ready for government are worried they are not as ready as they should be. Starmer has shown that he can be ruthless and, if he does become prime minister, there are likely to be nasty shocks for some now assuming they will be ministers in government. Labour already have about 20 more “front bench” spokespeople than there are paid ministerial jobs in government.

In Blair’s case, Mandelson says the “real” and “important” reshuffle shake-out took place after a year in office.

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The head of government has other unavoidable duties. Blair had to cope with the death of Princess Diana within months of being elected. “Why me?” Truss asked of the death of Queen Elizabeth II just days into her premiership.

Before the state opening of parliament on 17 July, the next prime minister will have to represent the UK at NATO’s 75th anniversary summit in Washington between 9 and 11 July with Ukraine at the top of the agenda. A week later he will host the European Political Community at Blenheim Palace – this organisation came into existence after Brexit to improve relations between 50 European nations.

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Prime ministers make what come to be seen as unforced errors – such as Truss’s mini-budget, Cameron’s Brexit referendum and Blair’s invasion of Iraq. They also have to cope with unforeseen events beyond their control, such as the COVID-19 pandemic.

The shocks of the first few weeks as prime minister are likely to be dwarfed by the shocks in the years to come. However hard they have tried to prepare, whether or not they are ready, the job will soon find out a prime minister’s strengths and weaknesses.