Mr McDonnell became the highest profile Labour MP to speak out on Jeremy Corbyn and Tom Watson’s replacements as he announced his support for Rebecca Long-Bailey as leader and Richard Burgon as deputy leader.
Left-wing MPs Ms Long-Bailey and Mr Burgon are close allies of Mr Corbyn and Mr McDonnell. They have been mocked for apparently being “continuity” candidates after the party’s election disaster last month.
Mr McDonnell told reporters in Westminster on Tuesday: “I’m supporting Becky, yeah.
“I thought she was superb on the Today programme (earlier on Tuesday). I thought she came across as an extremely warm human being and was on top of her game, she was excellent.
“But all of them have been good. I think we’ve got a really good team of candidates.”
He also promised to “keep out” of the campaign, joking he would only be “making pompous statements every now and then”.
Ayesha Hazarika, a former advisor to Mr Corbyn’s predecessor Ed Miliband, reacted to Mr McDonnell’s endorsement by tweeting: “And there we have it. Please Sir can we have some more?”
On the BBC Radio 4 Today programme mentioned by Mr McDonnell, Ms Long-Bailey said she “still supports Jeremy because I felt that he was the right man with the right moral integrity to lead the party”.
However, she also attempted to distance herself from Mr Corbyn’s leadership by saying Labour wasn’t “trusted” on Brexit, antisemitism and its manifesto policies.
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Ms Long-Bailey, one of six MPs who have declared their leadership candidacy, also said she would be prepared to use nuclear weapons as prime minister – something Mr Corbyn has campaigned against his entire political life.
Mr Burgon, meanwhile, on Monday night joked the “only time” he would be on a different side to Mr Corbyn is when his Leeds United plays against Mr Corbyn’s Arsenal.