Andy Burnham is facing a revolt over his push to alter Britain's voting system, with a Labour grandee warning it will allow parties which hold 'outlandish views' to call the shots.
The Prime Minister used his Labour Party conference speech last month to announce plans to contest the next general election with a manifesto pledging electoral reform.
He has previously argued in favour of replacing the current first-past-the-post system for Westminster contests with proportional representation (PR).
This would potentially make coalition governments more likely, increasing the influence of less popular parties such as the Greens.
During his speech, Mr Burnham also told Labour members he was creating a new national commission to look at the issue 'so that everyone's vote counts and everyone's voice is heard'.
But deep splits have since emerged within Labour's ranks over Mr Burnham's push to alter the way the UK elects MPs and governments.
Lord Blunkett, the Labour peer and former home secretary, urged supporters of electoral reform to be 'extremely careful' what they wish for.
Andy Burnham is facing a revolt over his push to alter Britain's voting system with a Labour grandee warning it will allow parties with 'outlandish views' to call the shots
It has been reported that Louise Haigh, who is Mr Burnham's de facto deputy PM, also has concerns about shifting to proportional representation
He pointed to Israeli politics - where the country's PM, Benjamin Netanyahu, is in coalition with ultranationalist allies who reject the idea of a two-state solution to the Israeli-Palestinian conflict - as a warning to Britain.
'If there were ever an example of how tiny minorities representing not only outlandish views but providing a blockage on progress, compromise and long-term outcomes, it has to be this form of PR,' Lord Blunkett wrote in The i Paper.
It has been reported that Louise Haigh, who is Mr Burnham's de facto deputy PM, also has concerns about shifting to PR, while Angela Rayner, the Housing Secretary, has previously voiced opposition to ditching first-past-the-post.
John Healey, who is now Chancellor, campaigned against electoral reform when voters were asked to approve the Alternative Vote system in a 2011 referendum.
But Ed Miliband, the Foreign Secretary, was on the opposite side in that referendum, and Energy Secretary Miatta Fahnbulleh was among Labour MPs to support calls for electoral reform earlier this year.
There are also divisions among Labour backbenchers, with Jonathan Brash, the MP for Hartlepool, saying he 'would never support a system that weakened that relationship between an MP and the people they represent'.
'I want to be the MP for Hartlepool,' he said.
Some Labour MPs have also expressed fears about having to rely on the Green Party or Liberal Democrats for support in a future coalition government elected under PR.