Electoral register 'down by 920,000'
The number of people on electoral registers in England and Wales dropped by 920,000 between March and December last year.
The Electoral Commission discovered the 2% reduction when it took a snapshot of local authority registers on 1 December.
It said people moving home and students were a particular challenge.
Ministers said the drop was because there was no "comprehensive" canvassing of households last year.
In a written statement, Gary Streeter, representing the Speaker's committee on the Electoral Commission, said 2014 had been "unique" and that from now on, forms would be sent to all homes to check who is resident and identify people who can been added to the register.
How to register to vote
You can register once you are 16 although you will only be able to vote on 7 May if you are 18 or over on the day.
If you are a citizen of another EU member state or Commonwealth country living in the UK, and unsure whether you are entitled to vote in the general election or local elections check the Electoral Commission website.
You can register to vote online through the government's gov.uk portal. The process takes five minutes and you will need your National Insurance number.
If you are unsure whether you are already registered or want to update your details, contact your local electoral registration officer to find out. You can also register to vote by post.
The commission said some of the reduction in entries could relate to people who were wrongly entered on the register.
It said the number of registered "attainers" - 17-year-olds who will be 18 by polling day - was "significantly lower than in previous years".
Problems in getting accurate data were highlighted, with the commission saying this had prevented it from making an assessment of the transition to the new system of individual voter registration.
But it said the new online registration system remained popular, with a satisfaction rating of 90%.
Shadow justice secretary Sadiq Khan said the figures confirmed Labour's "worst fears" about the way changes were being made to electoral registration and represented a "disaster for our democracy".