Sir Keir Starmer has been warned his plans to reform how Labour elects its leader could “rip our party apart”.
Sir Keir wants to scrap Labour’s one member, one vote system introduced in 2014 and return to the electoral college made up of unions and affiliate organisations, MPs and party members.
The plan has sparked opposition among sections of the party, just days before Labour gathers in Brighton for its annual party conference.
Mr Corbyn himself has described the move as “deeply undemocratic”.
Defending the change, Sir Keir said the current rules “focus us inwards to spend too much time talking to and about ourselves, and they weaken the link with our unions”.
But speaking to Kay Burley on Sky News, Mr McDonnell said his message for Sir Keir was: “Why rip our party apart?
“We’ll spend half our conference rowing amongst ourselves and it will spill over for the next six to 12 months.
“At the end of this conference we could be within 18 months of a general election.
“Do we really want to spend all this time having a row about the niceties and details of the Labour Party’s constitution?
“What we should be doing is being an effective opposition, holding the Tories to account.
“But also setting out that vision and the policy programme that we want to fight the next election on and the sort of society we want to create.”
The MP for Hayes and Harlington said Labour should be focusing on issues around the cost of living, with a “really tough winter” in prospect for many as a result of rising energy bills and the looming cut to Universal Credit.
“To be seen by the people of this country to then go into our conference – the first time we’re really meeting together physically for such a long time – and what are we doing? We’re arguing about the rules of the Labour Party, the constitution of the Labour Party,” Mr McDonnell continued.
“It’s a complete distraction. The problem that we’ve got in terms of the media presentation, you can see what’s happening.
“You have Boris Johnson strutting the world stage. I think he’s not doing a particularly good job, but he’s there with Biden, he’s preparing for COP26 [the climate summit], I think he’s failing to do that effectively.
“But contrast that with our own leader, who’s doing what? We’re having arguments over the constitution and rules of the Labour Party. It isn’t a good image.”
And he said Sir Keir ran the risk of “opening himself up to charges of dishonesty” with his plan.
“He was only elected 18 months ago as leader on a one member, one vote system,” Mr McDonnell said, referring to Sir Keir succeeding Mr Corbyn as leader in April 2020.
“He never said during that election that he wanted to change the rules. He’s opening himself up to charges of dishonesty in that respect.”