Voters in England, Scotland and Wales have until the end of Monday to register for May’s elections.
They are being urged to do so, amid a warning that up to 9.4 million eligible adults have not.
More contests than usual are taking place, with many elections having been delayed from last year due to Covid.
Voters will choose between candidates for the Welsh and Scottish parliaments, English councils, city mayoralties and police and crime commissioners.
Around 48 million people live in areas where elections are being held.
The Electoral Reform Society (ERS) has called for a system of automatic registration to drive up voter participation.
“It’s a scandal that nine million people are missing from the roll – and it hits young people, renters and some BAME groups particularly hard.” said Jess Garland, the campaign group’s policy director.
The ERS suggests trialling a system that would allow voters to register themselves on polling day itself, rather than in advance.
At the English local elections in 2018, the last for which data is available, voter turnout was just under 35%, according to the Electoral Commission.
A Cabinet Office spokesman said there were “no plans” to introduce automatic voter registration.
Election campaigning has been muted this year, thanks to pandemic restrictions and a pause following the death of the Duke of Edinburgh.