Boris Johnson is due to become the 55th person to be prime minister of Great Britain and Northern Ireland.
In charge of a budget of around £800bn, the UK’s 150,000 troops and the power to press the nuclear button, it is a role that carries supreme responsibility.
Like starting any new job however, a few administrative tasks must be completed before they can get to work.
So what is Mr Johnson facing on the first day in office?
One unusual aspect about the transition from one prime minister to the next is the speed at which it takes place. The UK is usually without a premier for around one hour.
The outgoing PM, in this case Theresa May, visits the Queen at Buckingham Palace to tender their resignation and recommends someone they believe can command the confidence of the House of Commons. (If the incumbent government has just lost an election, the outgoing PM will recommend the opposition party leader.)
The nominated successor is then summoned to the palace by the Queen’s private secretary and then Her Majesty invites them to form her next government in a tradition known as “kissing hands”.
Just before his appointment, former Labour Prime Minister Tony Blair was told by a Buckingham Palace official “you don’t actually kiss the Queen’s hand in the ceremony of kissing hands, you brush them gently with your lips” but it is thought a handshake will suffice.
Addressed as Mr or Ms before entering the Queen’s quarters, they leave with the formal title of prime minister thereafter.
Mr Johnson will become the 14th PM to serve her – Winston Churchill was the first.
Steps of Downing Street
While the world’s media wait back at Downing Street, a much enhanced security convoy and an armoured, bullet-proof Jaguar – the prime minister’s official vehicle – is on hand to whisk the new PM to their new home.
Heading straight to the lectern already in position, the new PM makes their first speech in the role. The words used can come to define the philosophy of a premiership.
Theresa May spoke of tackling the “burning injustices”, Margaret Thatcher recited part of a prayer and Gordon Brown recounted his old school motto: “I will try my utmost.”
Meeting the staff
Downing Street staff will be waiting to clap in the latest office holder – their hands still warm from clapping out the predecessor an hour before.
Though seemingly an inconsequential moment, it is important to make a good first impression.
“When Tony left we had champagne and then clapped him out. When Gordon arrived we clapped him in and then had coffee upstairs. It set the tone for the premiership,” said Theo Bertram, a former adviser to both Mr Blair and Mr Brown.
After shaking a few hands, the new PM heads straight to the cabinet room to be briefed by officials for the next few hours.
This includes the cabinet secretary – the UK’s top civil servant – whose advice ranges from day-to-day governing to expenses allowances and living arrangements. The new chief is not just changing jobs but moving house at the same time.
There will also be security briefing from the chief of defence staff, the national security adviser and the heads of the intelligence agencies with details of British spies and operations overseas as well as procedure involving Britain’s nuclear deterrent.
After a nuclear briefing, the newest occupant of No 10 will write their “letters of last resort” – instructing the chief commander of the four submarines which hold Britain’s nuclear arsenal what actions to take if the country is obliterated by a nuclear strike.
Letters are sealed – with the hope they are never opened – and the previous instructions are destroyed, nobody having read their contents. John Major described writing the letter as “one of the most difficult things I’ve ever had to do”.
The new PM will also have to nominate “nuclear deputies” – two other cabinet members who take charge of the codes in an emergency if the premier is unwell or unable to be reached.
Peppered throughout the day will be calls from other world leaders congratulating the newest member of their club – Barack Obama was said to have phoned David Cameron just 30 minutes after he first entered Downing Street.
Building a team
Though plans may have been in place for some time, the new prime minister now has to appoint a cabinet and ministerial team to head up their government departments.
Crucially, they also will sack incumbent ministers deemed surplus to requirements.
When drafting his cabinet, Gordon Brown reportedly wrote the names in pencil so they could be rubbed out and replaced, such was his deliberation. Whiteboards with stickers is the usual modus operandi.
In the first day key positions such as chancellor, foreign secretary and home secretary will be filled with more junior roles coming later.
Before any announcements, civil servants will be frantically vetting candidates to flag any conflict of interests.
Establishing a backroom team, a chain of command, planning the Queen’s Speech with a policy agenda and setting objectives for the first 100 days are all crucial next steps.
And then the hard work and tough decisions in one of the most challenging jobs on earth begin.
Boris Johnson: What can the new PM expect on his first day?}