John McDonnell has accused Sir Keir Starmer of acting like a monarch amid a row over whether Labour should adopt proportional voting.
Mr McDonnell, the former shadow chancellor and a supporter of a new voting system, accused the Labour leader of spending too much time mixing with the "royals" of their party.
Speaking on Sunday night at a rally organised by the Labour for a New Democracy protest group, Mr McDonnell said he hoped for "constructive dialogue" with the Labour leadership.
"I think we just have to remind [Keir], as gently as we can, that all our decisions are taken democratically," he said.
"I think Keir’s been mixing with the royalty too much for the last few days. He’s not a feudal monarchy.
"He’s an elected leader of the party, accountable to the party, and accountable to all of us."
A source told the Mirror newspaper that Sir Keir was "intensely relaxed" about the prospect of the motion passing, because there was nothing forcing him to make it party policy.
Andy Burnham, the mayor of Greater Manchester, is among other prominent Labour figures supporting the growing movement in the party to replace the existing first-past-the-post system.
Describing the current system as "antiquated", Mr Burnham said it had allowed the Conservatives to spend decades in power "without ever winning a majority share of the popular vote".
The campaign has also received the support of Unison, Britain’s biggest trade union, which passed a change in its stance at its own conference in summer.
But in an interview with Sunday’s Observer, Sir Keir said: "It’s not a priority for me… There are a lot of people in the Labour Party who are pro-PR.
"It’s not a priority and we go into the next election under the same system that we’ve got, first-past-the-post. And I’m not doing any deals going into the election or coming out of the election."
Labour faced accusations of making an informal "pact" with the Liberal Democrats in May after some candidates appeared to stand aside for one another at the local elections, but the existence of any arrangement was denied by both parties.