MPs demanded extra time for people to register to vote in the EU referendum after "unprecedented demand" caused the website to crash.
David Cameron was among senior figures who urged individuals to sign up before the midnight deadline but dozens were left frustrated.
A last-minute rush to sign up was stalled after the website failed, with Labour leader Jeremy Corbyn and Liberal Democrat leader Tim Farron among those demanding an extension.
Farron said it could be a "major blow" to the prospects of the UK remaining in the EU if young people were denied a say.
His party said all other Parliamentary business should be "put on hold" so that emergency changes could be rushed through.
The Government issued an online apology to those who experienced issues.
More than half a million people registered to vote on Tuesday, with just over a fortnight to go before the referendum on June 23.
A live usage site said more than 26,000 people were using the service with just five minutes to go before the midnight cut-off point, with more than 20,000 still on the site at 12.01am on Wednesday morning.
Traffic peaked at around 10.15pm when there were more than 50,000 people on the registration page.
The service struggled as voters flooded the site, with pages taking longer to load before the site appeared to go down. According to the usage site the service began to recover before the deadline passed.
The Electoral Commission - which oversees the running of the referendum - said it was aware of the issue but pointed out that the deadline was set by Parliament in legislation.
Some of those attempting to register found the message: "Sorry we are having technical problems. Please try again in a few moments."
Around 132,000 of the people who registered on Tuesday were aged under 25, compared to around 13,000 from the 65 to 74-year-old age group.
Student Jordan Parker, 20, said he began trying to register from 10pm but the website had timed out after he entered all his details. After 50 minutes he still had not been able to register.
He said: "It is annoying. They can't get anything right, can they? This is a massive vote for my generation, the website going down will affect more young people I would expect. They have got to extend the deadline."
Parker feared more young people would be affected by the website crashing, as they tended to be more "disorganised" and register later.