Video report by ITV News Political Editor Robert Peston
Watch ITV News special coverage as Boris Johnson becomes the new prime minister - live from 2pm on Wednesday on ITV and at itv.com/news
Just hours after being confirmed as the new leader of the Conservative Party, Boris Johnson will soon be making the key appointments for his first Cabinet as Prime Minister.
With less than 100 days to go before Brexit, the pressure is on Mr Johnson to bring together a team who will help him deliver his promises "do or die".
It's already known he'll be losing Chancellor Philip Hammond, and Rory Stewart has said he will stand down as International Development Secretary, while David Gauke will quit as Justice Secretary.
Who is expected to take seats at Johnson's top table?
Priti Patel is expected to take one of the top jobs, returning after scandal saw her resign from high office in 2017.
Westminster whispers say Sajid Javid is likely to get the job as Chancellor of the Exchequer.
After a "love in" from the new Tory leader, it's thought Jeremy Hunt will be kept on in Cabinet - but his role may change. Some have said Mr Hunt was offered the job of looking after defence, but declined on the basis of keeping his current office or nothing at all.
It has been suggested David Davis may also make a return, the former Brexit secretary could bring to the top table the clout Mr Johnson may need to ensure the United Kingdom leaves the EU by the 31 October deadline he reassured voters he would abide by.
Mark Spencer will become his chief whip, it was announced on Tuesday afternoon.
Mr Johnson will have to make some difficult decisions after meeting with the Queen at Buckingham Palace.
Speculation is mounting over whether leadership runner Jeremy Hunt will keep his Foreign Office position or be demoted to a lower ranking.
How did Boris Johnson become Tory party leader - and his he Prime Minister yet?
Mr Johnson won a six-week long campaign to become Conservative leader, beating Jeremy Hunt to guide his party.
He won by a clear majority, securing 92,153 votes (66.4%), with Mr Hunt receiving 46,656 (33.6%) and a high turnout of 87.4% of Conservative Party members.
Addressing the crowds in his first speech as Conservative leader, Mr Johnson said: "We are going to energise the country, we are going to get Brexit done on October 31st.
"We are going to take advantage of all the opportunities it will bring in a new spirit of can-do and we are once again going to believe in ourselves."
He added: "I will work flat out from now on with my team that I will build, I hope in the next few days to repay your confidence, but in the mean time the campaign is over and the work begins."
Mr Johnson said it was an "extraordinary honour and privilege" to be elected and was determined to defeat Jeremy Corbyn.
But winning the contest doesn't automatically mean he is Prime Minister. Mr Johnson will need to visit the Queen at Buckingham Palace first, there he'll ask permission to form a Government and will, at that point, become PM.
He'll face a challenging start to his time in office, as ITV News Political Correspondent Paul Brand explains.
What has the reaction to Mr Johnson winning the leadership race been?
US President Donald Trump congratulated Mr Johnson on winning the race to be the next prime minister, tweeting: "He will be great!"
Home Secretary Sajid Javid, who was eliminated in the fourth round of the Tory leadership contest, said on Twitter: "Well fought campaign by my fine colleague Jeremy Hunt.
"Congratulations Boris Johnson on a resounding victory! Now let's come together as a party under his superb leadership, so we can deliver Brexit, unite our great country and defeat Corbyn."
Jacob Rees-Mogg congratulated Boris Johnson on his "clear mandate" to lead the country.
He said on Twitter: "Congratulations to Boris Johnson on becoming Leader of the Conservative Party with 66% of the vote - a clear mandate to deliver Brexit and unite the country."
Former Tory leadership hopeful Michael Gove tweeted: "Congratulations Boris Johnson - you ran a brilliant campaign!
"Time for Conservatives to come together and deliver Brexit."
Speaking after his defeat, Mr Hunt said he was "disappointed" but "incredibly proud" of the campaign he and his team ran.
"This was always going to be uphill for us because I was someone who voted remain.
"We have a prime minister who voted remain and I think lots of Conservative Party members felt that this was a moment where you just had to have someone who voted for Brexit in the referendum," he said.
Mr Hunt added: "In retrospect that was a hurdle we were never able to overcome but I think, that being said, there was a lot of progress and I am delighted for the country that Boris has become prime minister, I think he will be a great prime minister."
While Brexit Party leader Nigel Farage wrote: "I wish Boris Johnson well as Prime Minister with his do or die pledge to deliver Brexit on October 31st.
"Does he have the courage to deliver?"
Prime Minister Theresa May tweeted her congratulations to Mr Johnson, but said the party must work together "to deliver a Brexit that works for the whole UK", a clear warning against a no-deal departure.
Fellow Tory leadership candidate Mr Hunt offered his congratulations to his opponent, saying he fought the contest with "optimism, energy and unbounded confidence."
Green party leader and MP for Brighton, Caroline Lucas, said: "Around 100,000 Tory party members have inflicted on us a prime minister with a record of bigotry, racism, lying and incompetence.
"This is not democracy. This is not in our name."
While EU Commission Brexit negotiator Michel Barnier tweeted he was ready to "work constructively" with the new prime minister.
However Mr Johnson's election as Tory leader has not been welcomed by all in the political sphere, with Labour leader Jeremy Corbyn tweeting the next Prime Minister should be decided by "the people of our country".
Mr Corbyn told ITV News the Labour Party will table a motion of no confidence "when appropriate to do so".
He added: "We will decide when that will be, it will be an interesting surprise for all of you."
The new prime minister will have to govern with a Tory-DUP majority of just two, after Dover MP Charlie Elphicke had the Conservative whip suspended when he was charged with sexually assaulting two women.
The Government majority could be further reduced next week if the Tories lose the Brecon and Radnorshire by-election.