Next week MPs will vote on whether there should be a referendum on switching to the Alternative Vote (AV) system after the next general election.
The AV system asks voters to numerically rank candidates in order of preference rather than placing an ‘X’ next to a single candidate.
Brown’s decision to campaign for an AV system will be seen as an attempt to woo the Liberal Democrats, who could play kingmaker in the event of a hung parliament.
Although Liberal Democrat home affairs spokesman Chris Huhne dismissed Brown’s championing of an AV system as a “deathbed conversion”, he said that most of his fellow Lib Dem MPs would probably back a referendum.
But Conservative chairman Eric Pickles said: “It’s not the voting system that needs changing, it’s this weak and divided Government.”