As many as 10 million voters, predominantly poor, young or black, and more liable to vote Labour, could fall off the electoral register under government plans, the Electoral Commission, electoral administrators and psephologists warned .

The changes will pave the way for a further review of constituency boundaries that will reduce the number of safe Labour seats before the 2020 election.

MPs on the political and constitutional reform select committee only realised the implications of the plans following three evidence sessions with election experts over the past week to examine the white paper which proposes to introduce individual electoral registration rather than household registration before the 2015 election.

The committee chairman, Labour MP Graham Allen, said they were "genuinely shocked". Even Tory members such as Eleanor Laing expressed surprise.

The policy has been described by Jenny Russell, the chair of the electoral commission, as the biggest change to voting since the introduction of the universal franchise.

Ministers have unexpectedly proposed that it should no longer be compulsory to co-operate with electoral registration officers (EROs) when they try to compile an accurate register, in effect downgrading the civic duty to engage with politics.

Russell warned: "It is logical to suggest that those that do not vote in elections will not see the point of registering to vote and it is possible that the register may therefore go from a 90%completeness that we currently have to 60-65%."

John Stewart, chairman of the electoral registration officers, said the drop-off was likely to be 10% in "the leafy shires" but closer to 30% in inner city areas. He said there would be an incentive not to register as the list is used for jury service and to combat credit fraud. He said he expected large numbers of young voters would not register.

The Cabinet Office, overseen by Nick Clegg, which had already decided there would be no household canvass in 2014 to save money, is introducing individual registration before the 2015 general election. The Electoral Commission said the change would mean 10% of the electorate could fall off the register in as many as 300 local authority areas.

The full effect of voluntary individual registration will be felt at the 2020 general election because the constituency boundaries for that election will be based on a voluntary individual register compiled in December 2015.

The projected 30% fall off in registered voters, weighted towards poorer voters, would require the boundary commission to reduce the number of inner-city Labour seats because the Boundary Commission is required to draw up constituencies with the sole objective of equalising the size of the electorates and not to take into account natural or political borders.

It is already estimated that as many as 3 million people currently eligible to vote do not register even though it is compulsory to co-operate with the compilation of the registry.

Although individual registration will be introduced before the 2015 general election, ministers have said the names on the existing household register can be carried over on to the election register, so reducing the impact.

Tristam Hunt, a Labour committee member, said: "These plans show how little this government really cares about democracy or fairness. If they get away with it, the effect on the 2020 general election will make the chaotic boundary review published this week look minor. This is designed to wipe the poor and the young off the political map.

"We are moving from a notion of registering as part as a civic duty to something akin to personal choice like a Nectar card or BA miles."

Russell said the government's plans had "unforeseen consequences".

It is currently an offence, liable to a maximum fine of £1,000, to fail to comply with a request for information from an ERO or to give false information.

The Cabinet Office white paper, published in the summer said: "While we strongly encourage people to register to vote, the government believes the act is one of personal choice and as such there should be no compulsion placed on an individual to make an application to register to vote."

Roger Mortimore from pollsters Ipsos Mori warned: "It is a very dramatic change and I am opposed to it. So far there is a political effect, it is most likely to disadvantage Labour", because "people that are least engaged in politics — the poor, the young and the ethnic minorities and all those groups, when they do vote at all are more likely to vote Labour".