A Brexit supporter holds a Union Flag at a Vote Leave rally in London, June 4, 2016. Reuters/Neil Hall

Britain's electoral watchdog called on the government on Wednesday to consider extending the voter registration period for a June 23 referendum on EU membership after a key website crashed shortly before the deadline.

More than a million potential voters have applied to register online over the last week, half of them on the final day, according to a government website providing data on the registration system.

Turnout is expected to be key to the outcome of the close-fought referendum, with young people considered to be more pro-EU but also less likely to vote. More than half of those who registered on the final day were under 34.

Less than half an hour before the midnight (7 p.m. ET) June 7 deadline to register, the government tweeted that it was aware of an issue with the website and urged people to keep trying as some were still getting through.

The Electoral Commission watchdog said on Wednesday it had asked for the deadline to be extended.

"The registration deadline is set out in legislation and we have said to the government this morning they should consider options for introducing legislation as soon as possible that would extend the deadline," it added in a statement.

The Cabinet Office, the government department in charge of the issue, said on Twitter it was "urgently considering" what options were available to those unable to register on Tuesday night.

Lawmakers from several parties, including opposition Labour leader Jeremy Corbyn and Liberal Democrat leader Tim Farron, also called for the deadline to be extended following the problems.

Labour spokeswoman on voter registration, Gloria De Piero, said the party would support any legislative changes needed to extend the registration period.

"The tens of thousands of voters who were trying to register when the voter registration site crashed ... could have a decisive impact on the result," she said in a statement.

"We need the government to act to give a 24-hour extension of both the voter registration and postal vote deadlines, so that anyone who has registered to vote today will be able to have their say in the decision of a lifetime."

The Guardian newspaper also reported thousands of postal votes from British citizens in Germany may have got lost in the post after confusion about the type of pre-paid envelope supplied.