The Shadow Chancellor told BBC Radio 5 Live incumbent leader Jeremy Corbyn should be succeeded by a woman, ahead of his main speech at the Labour party conference in Liverpool.
He said: ”The next leader of the Labour party has to be a woman."
Contenders for the top spot in Mr Corbyns Cabinet include Shadow Foreign Secretary Emily Thornberry, Shadow Home Secretary Diane Abbott, and Angela Rayner.
In a string of early morning media appearances, Mr McDonnell also warned against the risks of a second vote on Britains EU membership.
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He told BBC Radio 4's Today programme: "If we don't get a general election, then yes, we will go for a People's Vote."
Asked what the vote would be on, he replied: "Parliament will decide what will be on that ballot paper.
"We will be arguing that it should be a vote on the deal itself."
Pressed on whether he wanted the option of staying in the EU on the ballot paper, he said: "If we are going to respect the referendum it will be about the deal."
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REFORM: Workers could be entitled to up to £500 a year extra under McDonnell's plans
(Pic: GETTY)
“The next leader of the Labour party has to be a woman.”
John McDonnell
He said: "All the polling that we have seen is that the country is still pretty split down the middle.
"My big worry is that if we go for a referendum which is seen as just a simple re-run we could divide the country again, we could get almost the same result or if it's slightly different that people demand another referendum…
"I don't want to revive UKIP in any way or even the far-right.”
Mr McDonnell is set to reveal a raft of new economic measures in his speech to the Labour party conference on Monday afternoon.
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SLASHED: Bookies rate Corbyn's chances of being the next Prime Minister at 5/1
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As part of his plans, firms would have to put 1% of their shares into the fund every year up to a maximum of 10%.
He is expected to say: "Workers, who create the wealth of a company, should share in its ownership and, yes, in the returns that it makes.
"The evidence shows that employee ownership increases a company's productivity and encourages long-term thinking.
"We believe it's right that we all share in the benefits that investment produces."
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Since its conference began on Sunday, Labour has announced plans for worker representation on company boards and to make employers provide 10 days paid leave for domestic violence victims.
Further announcements are expected to include clampdowns on rogue landlords evicting tenants without good reason and a ban on new academy and free schools.
Carolyn Fairbairn, director general of the CBI, which represents big business in the UK, said Labour was "wrong to assert that workers will be helped by these proposals in their current form".
She said: "Their diktat on employee share ownership will only encourage investors to pack their bags and will harm those who can least afford it.
“If investment falls, so does productivity and pay."
The Conservatives said Labour's proposal was "yet another tax rise" that would make it harder for firms to "take on staff and pay them a good wage".
Bookies have slashed the odds of Theresa May winning any upcoming general election – with bookmakers.tv ranking Corbyn at 5/1 to become Prime Minister.
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