The Electoral Commission calls it the "biggest change" to voting since the start of universal suffrage in 1928. What has attracted the attention of the independent Electoral Commission, and the ire of academics, pollsters, electoral registration officers, the Electoral Reform Society and, belatedly, the Labour Party, is the Conservative-led government's proposal to switch from a system of household registration of voters, which is vulnerable to fraud and error, to a system of individual electoral registration (IER), in which, crucially, it will no longer be compulsory for members of the public to co-operate with electoral registration officers.

It sounds technical and bureaucratic, perhaps a little dull and boring, too. The press isn't interested. It is yet to be debated in the Commons. But it will have a profound effect on general election results and, by extension, the future of British democracy. Put simply, the Tories, aided and abetted by the Liberal Democrats, are rigging the rules of the electoral system to make it easier for themselves to win parliamentary majorities after 2015.

Off the register

How so? The switch from households to individuals, coupled with the lack of enforcement, will lead to a sharp decline in the number of registered voters (as happened in Northern Ireland when the switch was made there, in 2002). It is estimated that at present 3.5 million people eligible to vote (or one in ten) do not register to do so. According to Jenny Watson, chair of the Electoral Commission, under voluntary IER, the electoral register could go "from a 90 per cent completeness that we currently have to 60-65 per cent" - an astonishing ten million or more voters could just fall off the register.

All of the empirical evidence suggests that those who tend not to register to vote are drawn disproportionately from the ranks of the young, the urban poor and ethnic minorities. These eligible voters, by complete coincidence, tend to support Labour. The partisan impact of IER is compounded by the government's plan to reduce the number of constituencies, and make them equal in size based on the number of registered voters.

Last month, Her Majesty's Opposition finally woke up to the threat. The Labour deputy leader, Harriet Harman, in her concluding speech to the party conference on 29 September, pointed out how the proposal is "going to push people off the electoral register - deny them their vote, deny them their voice. The numbers are going to be huge." The Tories, she said, "hope it will help them win the election".

The Cabinet Office minister Mark Harper responded by accusing Harman of hypocrisy and pointing out that Labour, in office, had backed the idea of IER. Conveniently, this misses the point. Critics of IER are not objecting to the shift away from the outdated and patriarchal household system of voter registration; they are objecting specifically to the shift away from compulsion and the inevitable disenfranchisement of the most marginalised voters.

The Tories have form here. According to a paper published in 1992 by the political scientists Iain McLean and Jeremy Smith, the introduction of the poll tax in April 1990 accounted "for slightly more than one-third of the estimated one million people shortfall between the electoral register and the [official] estimate of the qualified population". The authors later concluded that this "shortfall" - of poorer, urban, Labour-leaning voters - cost Neil Kinnock the 1992 general election.

Ministers argue that the move from compulsion to volition should be uncontroversial. "It is not compulsory to vote in our elections and nor will we compel people, so it is sensible that registering to vote should also be a choice for the individual concerned," says the coalition's white paper, published in June. However, most of the academic experts on voter registration disagree. "You have only to look at the US to see where that approach leads you," says McLean, a professor of politics at Oxford. "Look at the flagrant disenfranchisement of black voters in Florida in the run-up to the 2000 election."

Sharp Tonge

Jonathan Tonge, professor of politics at the University of Liverpool, is scathingly critical of individual electoral registration. "It will be a disaster in terms of shrinking the electoral register and reducing the numbers of people voting in general elections," he tells me. "If you diminish the number of people registered to vote, you delegitimise the outcome of elections."

Ministers talk incessantly of "liberties" and "rights". But with those go responsibilities and duties. As Ed Miliband has argued and as the

academics agree, registering to vote, like filling in a census form, is a "civic duty", not a lifestyle choice. What we are witnessing is a brazen attempt to gerrymander the electoral system, a cynical exercise aimed at keeping the Conservatives in power for a generation.

Miliband, who is said to be "steamed up" about IER, is planning an Obama-style registration drive in universities and colleges; both the Labour leadership and Ken Livingstone's mayoral campaign have been in discussions on this subject with Arnie Graf, the Chicago-based community organiser who mentored a young Barack Obama in the 1980s. The Electoral Commission will set out its formal response to the white paper in the next fortnight. The coalition is planning to introduce legislation in the new year. Time is of the essence. Miliband must act swiftly if he is to drive this seemingly technical dispute to the top of the political agenda and cut through to the voters.

After the August riots, David Cameron spoke of a "sick society". Yet his own reform of the electoral system will lead to a sick democracy, with fewer registered voters and lower turn­outs. It is the biggest political scandal you've never heard of.