More than 400,000 people applied to register to vote in EU referendum during the 48-hour extended period granted by the government.
The initial deadline of Tuesday evening was pushed back until 23:59 BST on Thursday after the registration website crashed - preventing people from ensuring they would be able to vote on June 23.
Figures released today reveal 436,347 people applied during the extra time. 238,903 used the website on Wednesday and 191,508 on Thursday.
A further 2,768 people applied to register to vote using paper forms on Wednesday and 3,168 did the same on Thursday.
The total number of people who applied in the last four days alone was 1.2m.
Labour's shadow chancellor John McDonnell put the large number down to the work of Jeremy Corbyn and shadow minister for young people Gloria De Piero.
Writing on The Huffington Post, the Labour leader said it was young people "that will be most affected by the decision".
"I want young people to have their voice heard in the EU referendum on 23 June and in politics more generally," he said. "Young people will live with the consequences for the longest."
On Wednesday the government enacted an “emergency” law to extend the deadline for voter registration.
Cabinet Office minister Matt Hancock revealed a massive 214,000 people an hour were trying to register - much higher than the 74,000 peak at last year’s general election - when the site came down.
Some Brexit campaigners have complained the extension could unfairly favour the Remain camp, arguing the extension would see more young people sign up to vote.
Younger voters are seen as more likely to back EU membership.
However Michael Gove, the chair of the Vote Leave campaign, welcomed the move.
"People will only have one chance to vote on whether they share free movement of people with Turkey, so the more people who register to vote on June 23 the better and we welcome the extension of the registration deadline," he said,
"It is particularly important given how few young people normally vote and I hope that this election will be different."
The Electoral Commission has said it is advising the government to plan for turnout as high as 80%.
At last year's general election turnout was 66%. At the 2014 Scottish independence referendum, it was a huge 85%.