Boris Johnson has confirmed he will run to be leader of the Conservative Party.
The former foreign secretary told a conference in Manchester that he would enter the race to succeed Theresa May if a vacancy arises.
He declined to answer questions from journalists as he left the event.
The prime minister has promised to step down if parliament ratifies her Brexit deal.
But she is refusing to name a departure date until then.
Mrs May is immune to ousting by Tory MPs until December 2019 – after winning a vote of confidence last year.
Some backbenchers are trying to change the rules so another vote can be held more quickly, while others – including her former co-chief of staff – are calling for her to go.
Mr Johnson was picked as one of the leaders of the Vote Leave campaign for Mrs May’s cabinet when she took over in the aftermath of the 2016 referendum.
He was foreign secretary for two years, before quitting over the Chequers proposal – Mrs May’s plan for the future UK-EU relationship after Brexit.
The Uxbridge and South Ruislip MP followed then-Brexit secretary David Davis out of the cabinet.
According to one Tory MP, Mr Johnson’s leadership campaign has been three years in the making.
He did not enter the race for prime minister back in 2016, declaring hours before the deadline for nominations closed that the new leader “cannot be me”, citing “the circumstances in parliament”.
Other MPs tipped to run to become prime minister include Environment Secretary Michael Gove, Work and Pensions Secretary Amber Rudd, Foreign Secretary Jeremy Hunt, Commons Leader Andrea Leadsom and ex-Brexit secretary Dominic Raab.
International Development Secretary Rory Stewart also said he wanted to move “beyond my brief” and believed he could “help bring the country together”.
The UK is now on course to leave the EU by 31 October, or earlier if parliament ratifies a deal.