Brown has presented the plan, which would not take effect until after this year's general election, as a way to revitalise British politics after the MPs' expenses scandal.
But critics say his real aims are to reclaim the rhetoric of reform from the Conservatives, who are ahead in the opinion polls, while paving the way for a possible Labour alliance with the Liberal Democrats.
Such an alliance would be his best hope of staying in power if the Conservatives fail to win an overall majority in the coming election, which must take place by June.
But Brown's timing, 13 years after Labour came to power and months before an election, has left him vulnerable to Conservative taunts that the measure is an electoral ploy -- particularly as Labour had promised electoral reform as far back as 1997.
Analysts say Brown is trying to present Labour as the party of change facing off against advocates of the status quo, in a counter-attack against the youthful Conservative leader David Cameron and his party's Year for Change slogan.
(Brown's) strategy is somewhat incredible, but it is also, probably, Labour's only hope, said the influential Economist magazine in its widely read Bagehot column this week.
Labour politicians insist the plan is not a gimmick.
It's about doing the right thing, because we do need to restore trust in politics, Environment Secretary Hilary Benn told the BBC on Tuesday.The House of Commons is due to vote on Brown's proposal to hold a referendum by October 2011 to let voters decide whether to keep the current system for electing MPs or change it.
The problem for Brown is that he faces a race against time to get the measure not only through the Commons but also the House of Lords before the general election.
He could end up with an incomplete legislative process that would simply be scrapped by an incoming Conservative government, or grind to a halt in the event of an inconclusive election.
His plan is meant to appeal to the Liberal Democrats, whose backing will be vital to Labour if there is no clear winner.
But the proposal falls short of the proportional representation they want and it is unclear whether Brown's signal would be enough to lure them into an alliance with him.
Labour is divided on the subject of electoral reform. Benn acknowledged on Tuesday that the project had not moved forward since 1997 because of a lack of consensus in the party, and many analysts are sceptical that Brown's plan can galvanise the Labour rank and file.
It comes as too little, too late, and ends up satisfying no one, said Dan Leighton, senior research fellow at Demos think-tank.
(Editing by Steve Addison)