The new prime minister is due to make a statement to the House of Commons having addressed his new cabinet ministers for the first time.
Boris Johnson told the cabinet they had “a momentous task ahead”, as he repeated his commitment for the UK to leave the EU on 31 October.
On Wednesday, Mr Johnson gave key roles to leading Brexiteers.
The appointments saw Sajid Javid as chancellor, and Dominic Raab and Priti Patel return to government.
Mr Raab was made foreign secretary and Ms Patel is home secretary.
More than half of Theresa May’s old cabinet, including leadership rival Jeremy Hunt, quit or were sacked.
BBC political editor Laura Kuenssberg said there were whispers there could be a big offer for European citizens coming out of cabinet this morning.
Speaking to his cabinet Mr Johnson said: “As you all know we have a momentous task ahead of us, at a pivotal moment in our country’s history.
“We are now committed, all of us, to leaving the European Union on October 31 or indeed earlier – no ifs, no buts.
“But we are not going to wait until October 31 to get on with a fantastic new agenda for our country, and that means delivering the priorities of the people.”
He also told the room it was “wonderful to see this new team assembled here” which respects the “depth and breadth of talent in our extraordinary party”.
Mr Johnson also used his first speech as prime minister to reiterate his determination to take the UK out of the EU by the 31 October “no ifs, no buts”.
The UK was originally supposed to leave the EU on 29 March but the deadline was moved to 31 October, after MPs rejected Mrs May’s withdrawal deal three times.