One of Jeremy Corbyn’s senior aides, who wrote Labour’s last manifesto, has announced his intention to resign.
Andrew Fisher, head of policy, will leave his job by the end of the year “to spend more time with his young family”.
The Sunday Times claims Mr Fisher warned that Mr Corbyn would not win the next general election.
Mr Corbyn confirmed that Mr Fisher was leaving, saying it was a “very stressful and full-on job.”
Mr Fisher is said to have revealed his intention to quit last week, according to a memo seen by the Sunday Times.
The newspaper reports that he criticised Mr Corbyn’s team for their “blizzard of lies”.
Mr Corbyn told the BBC’s Andrew Marr Show, “I think he said that because he was extremely distressed at that point about whatever was going on in discussions within the office at that moment.”
The Labour leader described Mr Fisher as a “great friend” and someone who he had worked with for 15 years – including Mr Corbyn’s time as a backbench MP.
“He is a great writer, he’s a great thinker and he’s done a huge amount of work in the party.
“We get along absolutely very well and he has promised that whatever happens in the future he will be working with me on policy issues.”
In a statement seen by the BBC’s political correspondent Iain Watson, Mr Fisher said he is “choosing to prioritise” his wife and young son.
He plans to leave by the end of the year but says he will stay on should there be an autumn general election.
Mr Fisher said he feels “immensely proud about what we have collectively achieved”.
But he added: “The long hours, stresses and strains that inevitably come from working in this high pressure environment mean I haven’t managed to balance my commitments to my wife and young son.
“So after four years, I’m now choosing to prioritise them. I will stay on for any autumn general election, but will be leaving by the end of the year.”
It comes after Labour’s deputy leader Tom Watson called for unity on Saturday following a move to oust him at the start of the party’s annual conference.
An initial attempt had been made at the Labour National Executive Committee (NEC) on Friday.
Mr Corbyn later suggested the role should be reviewed instead, and his suggestion was backed by the ruling NEC.
He has since told the Sunday Mirror he would like to see the party have two deputy leaders “which reflects diversity within our society”.
Mr Corbyn told the newspaper one deputy leader would be a woman.
He added: “Tom is the elected deputy leader of the party and so has an important role to play.
“I work with him and he’s done very well on media reform, online gambling and exposing the way sugar has a deleterious affect on our lives.”