Andy Burnham has criticised Keir Starmer for opposing dumping the first past the post system as the way to elect MPs.
The mayor of Manchester said Labour should instead back proportional representation to keep the Tories out of power.
His comments came as the Labour conference in Liverpool is expected to vote in favour of a motion calling for the party to officially support PR.
Starmer has insisted he will continue to support first past the post, with one ally telling HuffPost UK: “There are many things keeping me awake at night: the vote on PR is not one of them.”
But Burnham told Sky News he wanted to see Labour “be bolder” in its message to the country.
He said: “I’m a little disappointed to hear the party say it’s going to rule out electoral reform. I would say to them ‘hear the mood of conference on that particular issue’.
“I hope conference will still vote in favour of that because we’ve got to maximise the chance of a Labour-led government.
“I would also say for people who don’t support the case for electoral reform, over the summer we have just seen the most graphic demonstration we’ve ever seen of how a small elite can manipulate the political system in their interests.
“That’s what we saw on Friday - this was a budget for bankers, this was a budget for Tory donors. That is what has happened in this country and this is what Labour can now say to the public: we’re going to change things, we’re going to reform the way votes work, we’re going to reform the way politics works and we’re going to make it work for you.”
The debate comes after a British social attitudes survey this week showed a majority of the public - including 61 per cent of Labour voters - now back proportional representation.