Hundreds of thousands of people are at risk of missing out on a vote in the EU referendum and crucial elections after being erased from registers, campaigners warned.
Official statistics showed almost 600,000 fewer names on December's registers compared with a year ago - with some of the most dramatic declines among young people and in more deprived areas.
It is the first such assessment since the full introduction of individual voter registration (IVR) in a bid to clamp down on electoral fraud by scrapping the system where "heads" of households submitted all the names.
Critics blamed the Government's decision to accelerate the change by 12 months for leaving huge numbers of people in limbo with important votes just four months away.
And Labour and the Liberal Democrats accused ministers of deliberate attempts to skew the political battleground in their favour as the latest Office for National Statistics Data will underpin changes to Commons constituency boundaries.
The ONS said there were 44,934,029 people on local government election registers and 43,478,635 on parliamentary registers, down around 1.3% on the last annual UK figure.
A separate study by the Electoral Commission found that 770,000 names were removed from the register in December when the transition period to IVR ended, a year earlier than originally planned.
The watchdog - which cautioned the Government against speeding through the switch - said it could not say how many of those were legitimate entries - as opposed to people who had moved house or otherwise changed circumstances.
But it said it was "likely" people were missing out and noted that a pre-deadline push by returning officers to sort out 1.9 million remaining anomalies had resulted in nearly a quarter (24%) registering under the new system.
The Commission also found a 40% drop in the number of registered "attainers" - teenagers set to reach voting age during the year - suggesting they were missing out now that it was not down to parents to include them.
It is urging town halls to engage in a further push by writing to all those potentially at risk and said it was engaged in a concerted effort to ensure registers were up to date and accurate by June.
Electoral Reform chief executive Katie Ghose said the slump "shows the danger" of the speeded-up process and called for an awareness drive and a "registration revolution" including the ability to sign up on polling day itself.
"With elections all over the country in just three months, far too many people are now in danger of missing out on their most basic civic right," she said.
"Any fall at this early stage of transition from household registration to IER is worrying, particularly if it begins to become a trend as the new system takes hold.
"The constituencies which saw the biggest drop are largely student seats and deprived areas - groups which are already under-represented. The areas with the biggest rise are largely wealthier areas.
"This patchy picture means electoral registration - and the number of parliamentary seats representing each area - is getting more unequal by the year."
"We can't have a democratic system based on a registration postcode lottery. And whatever the size of the fall, this evidence of growing social and age divides in our democracy could be hugely damaging if it carries on in the long run."
Shadow minister for young people Gloria De Piero said: "What's worse is that the Government are shamelessly taking this as an opportunity to redraw constituency boundaries based on an electorate that is far lower than it should be.
"This is another example of David Cameron and the Conservative Party trying to rig the system for their own political ends.
"They want to reduce by fifty the number of directly elected Members of Parliament under the guise of reducing the cost of politics, whilst cramming the Lords at taxpayers' expense."
Liberal Democrat spokesman Tom Brake said:"There were already millions of people who were not on the register before this move - this has just made that problem worse. Individual Electoral Registration is a system worth moving to, but it shouldn't have been rushed and the Government should have followed Electoral Commission advice.
"It is another example of the Tories trying to stitch up the system in their favour."