The Labour leader could approve plans to replace FPTP with proportional representation (PR) next month.
This follows the decision by two of Labour’s powerful trade union backers to support electoral reform. Such a move could see the Tories locked out of power for a generation under a “coalition of chaos”.
Conservative MP Richard Holden, said: “Sir Keir Starmer knows Labour could never win a majority under his lacklustre leadership and Labour’s union paymasters know it too.
“That’s why they are calling for unity with the Lib Dems to change the rules so that Labour can sneak in, propped up by the Lib Dems and SNP, via the back door.”
He added: “Rather than trying to fiddle with Britain’s democratic system, the public would rather Labour and the unions concentrated on preventing strike action that cripples our public services.”
Tory fears over PR will embolden Labour activists who are pushing for their party’s annual conference next month to vote to include a switch to proportional representation in Labour’s next General Election manifesto.
The move follows a similar motion at last year’s conference which only failed to pass after opposition from the unions.
Unite – Labour’s biggest donor – now backs PR, as does Unison.
Andy Burnham, the Labour mayor of Greater Manchester and a potential successor to Sir Keir, has called for the party to endorse the controversial voting switch.
Earlier this summer, the Labour leader himself said he understood how many party members feel very strongly about PR – not least as he joined the party in East Surrey, where “every time you vote Labour, the vote doesn’t really count for anything”.
Under a draft motion for next month’s conference, the Labour For A New Democracy campaign calls on the next Labour government “during its first term in office” to scrap the “rotten electoral system which consistently hands power to a Conservative minority”.
However, Tory critics say it would simply lead to weak coalition governments where, after each Election, political parties would haggle over who to share power with.
Labour sources stressed that a party conference vote would not bind Sir Keir as to what went in Labour’s next manifesto.