The UK is facing a "silent, growing crisis in our democracy" over the decline in the number of people registered to vote, David Cameron has been warned in a letter from more than 50 politicians, academics and campaigners.
In the 1950s, an estimated 96 per cent of people were on the electoral roll, but this has fallen to 85 per cent with up to 7.5 million names missing. The letter says up to a third of young people are not registered.
The introduction of individual electoral registration (IER) - previously the "head" of a household could blockregister the family and universities could register everyone in a student hall of residence - has been blamed by some for increasing this trend.
Last week it was reported that 800,000 names had dropped off the electoral register since IER was brought in, although the Government said that number would have included people who had died or never existed.
The letter, signed by the leaders of the Liberal Democrats, Green Party and Plaid Cymru, the Bite the Ballot campaign and the Electoral Reform Society, says:
There is a silent, growing crisis in our democracy. The number of eligible voters missing from electoral registers has grown dramatically.
Reforms recently made to the electoral registration process to combat electoral fraud have led to many further names being removed from the electoral register. Worryingly, many would-be voters were turned away from the polls at the 2015 general election because they were not registered.
The Cabinet Office said it was "vital" to register as many people as possible and hoped efforts to increase the numbers could be done on a cross-party basis.