
The deadline passed at midnight (2300 GMT), and half a million people registered online on the final day, according to a government website that provides data on the voter registration system.
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But at 11:35 pm, the Cabinet Office, the government department in charge of the issue, tweeted that it was aware of an issue with the website, which was due to unprecedented demand, and was working to resolve it.
Minutes later, it tweeted an apology to those experiencing problems, urging them to keep trying as some people were still getting through. Almost an hour after the official deadline, it tweeted that the issue had been fixed, without specifying whether any applications received after midnight would be valid.
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Jeremy Corbyn, leader of the opposition Labour Party, said on Twitter: "I'm told (the official website) has crashed so people can't register to vote for #EUreferendum. If so, deadline has to be extended."
I'm told https://t.co/qXdulxPFk2 site has crashed so people can't register to vote for #EUreferendum. If so, deadline has to be extended
— Jeremy Corbyn (@jeremycorbyn) June 7, 2016
He was one of several high-profile politicians making similar calls. Asked whether some people had been unable to register on time, a Cabinet Office spokeswoman said: "We tried to resolve cases where people tried to register but were not able to." She gave no further details."
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