The Government is branded “a shambles” amid claims the failure could be a “major blow” to the UK’s prospects of staying in the EU.
MPs have called for an extension to EU referendum voter registration after “unprecedented demand” caused the website to crash.
More than half a million people registered to vote on Tuesday, with just over two weeks to go until Britain decides whether to stay in the European Union or go it alone on 23 June.
Traffic to the Government’s voter website peaked at around 10.15pm, with more than 50,000 people on the registration page – but many were left disappointed after the service failed during a last-minute rush to sign up.
The website was struggling, with pages taking longer to load as it was inundated with applications, before it appeared to go down altogether. The service appeared to recover shortly before the deadline passed.
A live usage site said more than 26,000 people were online with five minutes to go before the midnight deadline – and 20,000 were still on the site at 12.01am on Wednesday morning.
Some of those trying to register found the message: “Sorry we are having technical problems. Please try again in a few moments.”
Labour leader Jeremy Corbyn and Liberal Democrat leader Tim Farron were among those demanding an extension.
Mr Farron said the Government had presided over “a shambles,” which 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 an extension could be authorised.
Mr Corbyn tweeted: “I’m told (the) site has crashed so people can’t register to vote for #EUreferendum. If so, deadline has to be extended.”
The Government issued an online apology to those who experienced issues.
A Cabinet Office spokesperson said: “We became aware of technical issues late on Tuesday night due to unprecedented demand.
“Some people did manage to get through and their applications were processed. We tried to resolve the situation as quickly as was possible and to resolve cases where people tried to register but were not able to.”
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.
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 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.”