More than a million adults may be unable to vote if a general election is held this autumn, The Association of Electoral Administrators has warned.
Mr Brown has not ruled out an autumn election
An out-of-date electoral register would disfranchise people who have recently turned 18 and people who have moved home since last December, it said.
Speculation is high that Gordon Brown will call a snap election in November, having come to power in June.
But the updated register will not be published until 1 December.
The Conservatives, Labour and Liberal Democrats all say they are ready to run a campaign.
Mr Brown has refused to rule out an autumn election and speculation has been mounting that an announcement is imminent - as he is thought to be considering capitalising on Labour's lead in the opinion polls.
The date thought most likely for the prime minister to choose for an election is 1 or 8 November.
But AEA chief executive John Turner told the BBC: "The register is the main difficulty of itself.
"It was published on 1 December 2006, so it by then [the time of a November election] will be 11 months old."
He added: "And a lot of people who have moved since last December will unfortunately not be on the register for this election."
Mr Turner said the number of people unable to vote could be "upwards of a million people".
He advised anyone who has recently moved to contact their local council offices and ask for a "rolling registration form".
"So long as they have go that application and it's a valid application 11 working days before polling day, yes they will be added to the register."
Mr Brown has refused to rule out holding an autumn election. Mr Turner urged him to "keep your hand away from the phone".