More than 3.1m people have applied to register to vote since MPs agreed to an election at the end of October.
The Electoral Reform Society says 875,300 more applications have been made in this period, compared to the 34 days between the calling of the election and polling day in 2017.
It says the "surge" in registrations is "highly encouraging".
People who want to take part in the general election on 12 December had until midnight to register.
Those in England, Scotland or Wales who want to apply to vote by post had until 17:00 GMT on Tuesday to do so.
People who want someone to vote on their behalf have until the same time on 4 December to apply.
However the deadline for voters in Northern Ireland to apply for a postal or proxy vote has already passed.
About 45.8m people were already registered to vote in UK parliamentary elections as of December 2018, according to the Office for National Statistics.
About two-thirds of those who have registered since the end of October are under the age of 35, and more than a million are under 25.
There was a spike in registration among all age groups on 30 October, the day after MPs gave their backing to the pre-Christmas poll.
Out of 177,000 registrations on that day, 115,400 were from people aged under 35, according to the government website that tracks registration.
The next big spike came on 12 November, which coincided with a Labour Party Facebook campaign aimed at getting young people to register.
And another was on Friday, when nearly 308,000 registrations were recorded - the same day as BBC One's Question Time featured Boris Johnson, Jeremy Corbyn, Jo Swinson and Nicola Sturgeon in a two-hour special.
The biggest spike in applications so far was on Monday, the day before the registration deadline, when there were more than 366,000 registrations recorded - 150,000 of these were from people aged under 25.
An increase in applications is not firm evidence of an increase in the number of people able to vote - previous elections have seen voter registration applications from people who are already registered or who are below the voting age.
By Tom Barton, political correspondent
During the 50 days before voter registration deadline day, more than a third of applications, 1.3 million, were from under-25s. And while that is a significant increase on the 2017 figure of nearly 900,000, it should be seen in the context of an overall boost to the numbers of people applying to register. As a proportion of overall applications it is almost exactly the same as in 2017.
However, there has been a fall in the proportion of applications coming from 25-34 year olds, a group that was seen by experts as key to Labour's result two years ago. In 2017 33% of applications were from this group, this year it's below 30%.
Conversely, there's been an increase in the proportion of applications by voters over the age of 45, who make up nearly 20 percent of the total this year, compared with 16.5% in 2017.
So - while nearly half a million young people applying to join the electoral register might initially look like good news for Labour, the detail suggests the figures might not be quite as helpful for them as they first appear.
It comes as celebrities use their social media platforms to encourage people to register ahead of the deadline.
On Instagram, a video by the Game of Thrones actress Emilia Clarke urging people to register had been viewed over 5.7m times by Tuesday morning.
And on Twitter, a post by Manchester City and England footballer Raheem Sterling, including a link to the government registration page, had been "liked" more than 30,000 times in four hours.
‼‼‼ My guys this is a public service announcement.— Raheem Sterling (@sterling7) November 26, 2019
The deadline to register to vote in the General Election is 23:59 today! In one of the most important elections of our generation, make sure you have your say 🤟🏾
It takes just 5 minutes... REGISTER NOW https://t.co/QXOsvhXWYa