Theresa May has said she wants to lead the Conservatives into the next general election, telling the BBC she intends to remain in power "for the long term".
Speaking in Japan, the PM said her job was not just to deliver Brexit but to define the UK's place in the world and also to tackle domestic "injustices".
Some reports had suggested she could stand down in 2019 after EU withdrawal.
Foreign Secretary Boris Johnson gave Mrs May his "undivided backing", but Labour accused her of being delusional.
The prime minister has been under pressure after losing her Commons majority in a snap election called earlier this year.
Mr Johnson, speaking on a visit to Nigeria, said she could "certainly" win an absolute majority at the next general election.
But shadow Cabinet Office minister Jon Trickett said the prime minister was "deluding herself" about her plan to stay in power until the next election.
"Neither the public nor Tory MPs believe her fantasy of staying on till 2022," he said.
"Theresa May leads a zombie government."
The next general election is not scheduled to take place until May 2022, by which point Mrs May - if she stayed in Downing Street - would have been PM for nearly six years.
In the immediate aftermath of her party's failure to win June's general election outright, several MPs called on her to consider her position.
Former Chancellor George Osborne, who has become a newspaper editor after being sacked by Mrs May, said she was a "dead woman walking".
The PM has sought to consolidate her position by negotiating a governing agreement with the Democratic Unionists and overhauling the way Downing Street works, replacing key advisers.
But this has not stopped speculation about how long she might remain in No 10 and about potential successors, although one cabinet minister earlier this summer blamed such talk on too much "warm prosecco".
Asked whether she wanted to lead her party into another general election, whenever that takes place, the prime minister told the BBC's Ben Wright in Kyoto that that was her intention.
"Yes, I'm here for the long term. What me and my government are about is not just delivering on Brexit but delivering a brighter future for the UK.
She said she wanted to ensure "global Britain" could take its trading place in the world, as well as dealing with "those injustices domestically that we need to do to ensure that strong, more global, but also fairer Britain for the future"
The prime minister faces a crucial few months with a number of tests of her authority within the party, including her second conference speech as party leader in October and key Brexit votes in the Commons.
Newspapers reports over the weekend claimed Mrs May had told MPs that she intended to stand down in the summer of 2019 to give her successor ample time to bed in before the next election.
No 10 dismissed the reports as "peak silly season".
Mr Johnson, who received public backing from Mrs May after recent criticism of his performance, said: "I've made it clear I'm giving my undivided backing to Theresa May.
'She gets it'
"We need to get Brexit done.
"She's ideally placed to deliver a great outcome for our country and then deliver what we all want to see, which is this exciting agenda of global Britain.
"I think she gets it. She really wants to deliver it. I'm here to support her."
The UK is due to leave the EU in March 2019 and supporters of Mrs May have said leadership speculation only serves to undermine attempts to secure the best possible terms of exit.
On the second day of her trip to Japan, Mrs May will hold official talks with her counterpart Shinzo Abe and emphasise the growing security links between the two countries.