Ken Livingstone has bowed to pressure and apologised to Labour MP Kevan Jones for calling him "disturbed" and in need of "psychiatric help".
The former London mayor had repeatedly ducked chances to say sorry to the shadow defence minister, who has suffered from depression.
But after party leader Jeremy Corbyn ordered him to apologise, Livingstone eventually tweeted:
1/2 I unreservedly apologise to Kevan Jones for my comments. They should not have been made at all, let alone in this context.
2/2 I also make this apology because Jeremy is right to insist on a more civil politics and as a party we should take this seriously
Livingstone had hit out at Jones, MP for Durham North, after the latter questioned whether the ex-mayor should be co-chairing a review of Labour's defence policy.
It emerged on Wednesday that Livingstone, who has long campaigned against the Trident nuclear deterrent, will work alongside shadow defence secretary Maria Eagle on the review.
Jones told Politics Home: "I'm not sure Ken knows anything about defence. It will only damage our credibility amongst those that do and who care about defence."
Responding to those remarks, Livingstone told the Daily Mirror. "I think he might need some psychiatric help. He's obviously very depressed and disturbed. He should pop off and see his GP before he makes these offensive comments.”
Livingstone was immediately savaged by Labour MPs for the comments, which came after Jones revealed his battle with "deep depression" in the House of Commons in 2012.
A spokesperson for Corbyn said: "Jeremy is incredibly concerned that people with mental health problems shouldn't be stigmatised. He has worked with Kevan in the past on this issue and is impressed by his bravery in speaking out on his own mental health issues. Ken should apologise to him straight away."
Yet at first Livingstone refused to apologise to Jones, telling LBC Radio: "He was rude about me, I was rude back to him, he needs to get over it." Asked when he would say sorry, Livingstone replied: "Don't wait for it."
Later he softened, telling BBC Radio 4's World at One: "I had no idea that he [Jones] had any mental health issues, otherwise I would never have said it. If he's upset, I'm sorry. I grew up in south London. If someone was rude to you, you were rude back. I didn't go to Eton with all that smarmy-charmy education."
Jones pointed out on Twitter that he went to a comprehensive school in Worksop.
He told the Daily Mirror that Livingstone's comments were "gravely offensive, not just personally but also to the many thousands who suffer from mental illness".
Luciana Berger, shadow minister for mental health, had also urged Livingstone to "apologise without delay". She said: "An individual's mental health should never be the excuse for insults, jibes or political point-scoring. These comments should be treated as seriously as racism or sexism."
Labour MP Chuka Umunna said he was "disappointed" with Livingstone.
Prejudice comes in many forms - very disappointed to see the man I campaigned for to be Mayor seemingly spreading it https://t.co/ksT9IQvZOF
Jones wasn't the only Labour MP to raise concerns over Livingstone's appointment. John Woodcock, whose Barrow and Furness constituency includes the shipyard that would build the Trident replacement submarines, didn't hold back on Twitter.
Every new day I think we have much reached the summit, but no: there always remains a fresh provocative absurdity.
Friends of Eagle, who supports the renewal of Trident, insisted to BuzzFeed News that she was still in charge of the review, despite Livingstone's status as "co-convenor". One Labour backbencher told us: "Pray for Maria."
In a recent interview with Russia Today, Livingstone said Eagle was "mad" for backing Trident, adding: "Most Labour MPs don't actually believe that Trident's worth spending £20 billion on."
Meanwhile Sir Lawrence Freedman, emeritus professor of war studies at King's College London and a member of the Chilcot Iraq war inquiry, tweeted:
Interesting that Ken Livingstone to co-chair Trident review with Maria Eagle. Lab clearly grasped concept of mutual assured destruction.
The row comes days after Corbyn was heavily criticised by his own MPs over his response to the terror attacks in Paris. He was forced to clarify on Tuesday that he did believe police should shoot terrorists dead if lives were at stake.
Corbyn has also faced anger over his refusal to criticise the Stop the War Coalition, which blamed the Paris attacks on "Western support for extremist violence" in the Middle East.
On Sunday, Livingstone said he had sympathy with that view, telling BBC Radio 4: "It’s the endless interventions of Britain and America and France in Arab countries that has come back to haunt us."
Labour MP Wes Streeting pointed that interview out on Twitter when Livingstone's appointment was announced.
Is this the same Ken Livingstone co-chairing Labour's defence review? https://t.co/LnmiOsITNV
Livingstone, a close ally of Corbyn for three decades, also angered MPs last month by warning that those who rebel against the new leader should face de-selection.
The defence review will make recommendations about Trident and other matters which will be put to Labour's National Policy Forum before being taken to party conference next autumn.
Corbyn has long opposed the like-for-like replacement of Trident but hopes to win over Labour MPs before a key Commons vote on the issue in the coming months.