Theresa May insisted on Sunday that she remained on course to be a long-term British prime minister as she prepared to reshuffle her cabinet following the sacking last month of one of her closest allies.

Mrs May said that she was “not a quitter” and intended to lead her party into the next national elections despite a series of high-level ministerial departures and a difficult first round of Brexit talks with the European Union.

The prime minister said she would “soon” be making changes to her ministerial team with her leadership strengthened after striking an interim deal with the European Union in December. The deal is a first step towards discussing a new trade relationship with the EU when Britain leaves the 28-nation trading bloc as expected in March 2019.

Boris Johnson, the flamboyant but gaffe-prone foreign secretary, was expected to keep his job after the reshuffle, according to newspaper reports on Sunday, along with her finance minister, Philip Hammond, and Brexit supremo David Davis.

The changes were expected to be focused on lower-tier cabinet members to be ousted in favour of a group of younger women and non-white MPs to better reflect modern Britain, according to the Sunday Times.

One of those tipped to lose her job in the cabinet was Andrea Leadsom, a staunch Brexiteer who fought against Mrs May in the ruling Conservative party’s leadership battle in 2016.

The reshuffle was expected following the departure of Damian Green, her effective deputy, in December. Mr Green was sacked after he made misleading statements about what he knew about pornography found on his office computer in 2008.

“There are no prizes for guessing Damian Green’s departure means some changes needed to be made and I will be making some changes,” she told the BBC.Mrs May’s disastrous decision to call national elections in 2017 to strengthen her hand before Brexit talks wiped out her party’s small majority and left her reliant on a small Northern Irish party to pass legislation. The electoral disaster raised doubts that she would be able to reach the end of 2017 in charge of her party.

But her fortunes have been revived by the deal struck with the EU and likely challengers have stepped into line behind her before the crucial second phase of Brexit talks scheduled to start in March. She said that she wanted to fight the next election as leader of her party.

“I am not a quitter, I am in this for the long-term,” she said in the interview broadcast on Sunday. “I have said I will serve as long as people want me to serve.”

Mr Johnson – seen as a potential challenger to Mrs May – was tipped to keep his job despite calls for him to be sacked over comments he made to MPs about a British-Iranian national imprisoned in Iran.

The family of Nazanin Zaghari-Ratcliffe insisted that she was in Iran to visit her family when she was arrested, but Mr Johnson indicated that she could have been involved in training journalists, a statement seized on by the Iranian authorities to justify her detention. Mr Johnson later apologised.