With just over five weeks to go before the general election, around 450,000 eligible voters still haven't enrolled.
More than half of them are under 30, the Electoral Commission says.
The deadline for enrolment isn't until the day before the country goes to the polls on September 23.
However, the commission is encouraging people to be enrolled by next Wednesday.
Chief electoral officer Alicia Wright says those enrolled by then will be sent an EasyVote pack with all the necessary information to make voting easier.
People enrolled after August 23 - which is also the cut-off for being included on the printed electoral roll - will have to cast a special declaration vote.
"You can still enrol right up until the day before election day, including at any advance voting place, but you'll need to make a special vote," Ms Wright said.
"A special vote takes a bit longer and means filling in an extra form. That's why we encourage people to get enrolled now."
An estimated 3.57 million people are eligible to vote in the election.