Jeremy Corbyn has sought to play down divisions within his top team after one of his closest aides said he would quit and criticised the party’s leadership.
Andrew Fisher’s exit comes after a failed bid to oust deputy leader Tom Watson, as Labour conference begins.
Mr Corbyn said he got on well with both men and Mr Fisher was “extremely distressed” when he wrote a memo saying the leader’s office was “incompetent”.
He said he would serve a five years if elected PM, adding: “Why wouldn’t I?”
In a wide-ranging interview with the BBC’s Andrew Marr, he dismissed talk he could stand down as Labour leader in the next year or so as “wishful thinking”.
He also defended his Brexit policy – to be debated later on Sunday – amid calls for him to come out unambiguously to remain in the EU.
Mr Corbyn was dealt a blow on Saturday when it emerged one of his aides, head of policy Andrew Fisher, revealed he will quit his post by the end of the year.
He said he wanted “to spend more time with his young family”, but the Sunday Times claims he resigned after warning that Mr Corbyn would not win the next general election.
Mr Corbyn acknowledged Mr Fisher, who helped write the 2017 manifesto, had expressed concerns about the party’s direction and he had spoken to him “at length” about it.
He said Mr Fisher was “extremely distressed” when he made the comments, suggesting it was the sort of disagreement which happened in many workplaces.
He also said Mr Fisher would continue to advise him when he left his official role.
Amid continuing fallout from the bid to oust Mr Watson, Mr Corbyn also said he was not told beforehand of Friday’s move by left-wingers on Labour’s ruling body to abolish the role.
The party will now consult on replacing the single role with two deputies – one of whom will be a woman.
Mr Corbyn, who has been at odds with Mr Watson over Brexit, said he got on “absolutely fine” with him and suggested his intervention had “put the issue to bed”.
On the second day of its conference, Labour is unveiling plans to scrap Ofsted and replace it with a new school inspection system on day two of its party conference.