More than 430,000 people applied to register to vote in the EU referendum during the extended deadline period.
The two-day extension was granted after the government website for registering voters failed just before Tuesday's original deadline.
A prominent Leave campaigner has said he is considering launching a legal challenge to that decision.
Leave.EU founder Arron Banks said there were grounds for a judicial review of the "unconstitutional" move.
The government had pushed through emergency legislation to allow people to register until 23:59 BST on Thursday.
The extension to the deadline covered everywhere apart from Northern Ireland, where the online system was not in use. The referendum on whether or not the UK remains in the EU will be held on 23 June.
Cabinet Office minister Oliver Letwin said in the hour leading up to the crash that there had been 214,000 registration applications.
Tuesday's rise was "three times as intense a spike as occurred before the general election" he said, and it would have taken a spike "six times as large" to cause the site to fail again, he told MPs.
According to the official government website, there were 242,000 applications to register to vote on Wednesday, the second highest total since mid-May when a registration campaign was launched. Of these, 135,600 were from people aged under 35.
There were a further 195,000 applications on Thursday, taking the overall two-day total to 437,000 applications.
The proportion of applications coming from younger age groups continued to be very high, as it was in the days before the deadline - 77% of those applying were from the youngest three age groups, meaning they were under 45.
'All legal options'
Mr Banks, an insurance millionaire, told BBC Radio 4's Today programme on Thursday: "We've got lawyers that are looking at [the extension] at the moment.
"They are tending to say it's unconstitutional because once you've set the rules you can't really change it halfway through, and Parliament really shouldn't be doing this."
BBC political correspondent Ross Hawkins said many Leave campaigners saw the deadline extension as a "fix" because they think people who signed up late will be younger and therefore more likely to support the EU.
The official leave campaign - Vote Leave, in which Mr Banks plays no part - has said the government was trying to register as many likely Remain voters as possible, but stopped short of suggesting that it would consult lawyers.