Prime Minister David Cameron has urged more people to register to vote in a referendum on Britain's EU membership after a technical glitch left thousands of people in limbo at the registration deadline of midnight on Tuesday.
"If you aren't registered and you want to vote in this EU referendum you should continue to register," Cameron said on Twitter.
"Huge demand caused a system overload last night," he said, adding that "urgent discussions" were taking place with the Electoral Commission about allowing more people to register.
The Electoral Commission said technical problems affected the final two hours of registration for the referendum on June 23.
It urged the government on Wednesday to "consider options for introducing legislation as soon as possible that would extend the deadline."
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Opposition leaders including Labour's Jeremy Corbyn have also called for the extension of the deadline for registration.
Some 45 million people are eligible to vote in the referendum, but many people are not expected to register or may register but not use their vote.
About 1.6 million people have registered online since the government began its campaign on May 15.
Several experts have forecast turnout of around 60 per cent, but Sky News on Wednesday quoted an electoral official as saying local authorities have been asked to prepare for a turnout of up to 80 per cent.
Voters will be asked the single question: Should the United Kingdom remain a member of the European Union or leave the European Union?
