More than 1.35 million people have applied online to register to vote in the past few weeks, the Electoral Commission said.
The applications have been made since a public awareness campaign about voting was launched on May 15 by the commission, in the build-up to the EU referendum.
Under-25s and 25 to 34-year-olds have been the most active groups, making up more than half of the online applications, with 763,183 submissions in the past few weeks.
Last Friday saw the biggest number of applications in a single day, the figures from May 15 to June 5 show, with 186,255 submitting an online application.
The Electoral Commission is warning people it is their last chance to register to vote in the referendum on June 23.
Jenny Watson, chief counting officer for the EU referendum, said: "If you aren't already registered to vote, you need to do it by the 7 June deadline or you won't be able to take part in this historic referendum.
"Tens of thousands of people applied to register the day after the deadline to register for the general election and were too late to have their say. Don't let that be you."
The number of applications does not necessarily reflect new voters, as those applying can include people who are already registered or those not eligible to vote.
People can apply to register to vote before the deadline of 11.59pm on Tuesday June 7 at gov.uk/register-to-vote.