David Cameron has urged would-be voters to continue registering beyond deadline after website crashes as ministers weigh up a law change which would allow late-comers to vote.
The registration deadline for people wishing to vote in the June 23 poll passed at midnight but a website crash potentially denied thousands the chance to take part.
The prime minister urged people to continue to register today, telling the House of Commons they were working with [the Electoral Commission] to make sure those who are registered today and those last night will be able to vote.'
Matt Hancock, the Cabinet Office minister, said that following discussions with the Electoral Commission and cross-party discussions secondary legislation would be sought to extend voter registration until midnight tomorrow (Thursday, June 9).
He said: 'Having taken the decision today, we think it is right to extend to midnight tomorrow (9 June) to allow people who have not yet registered time to get the message that registration is still open and get themselves registered.'
But senior Conservative MP Bernard Jenkin said such legislation would make Britain look like an 'absolute shambles'.
Mr Jenkin, chairman of the Public Administration and Constitutional Affairs Committee, said the Government and the Electoral Commission were 'ill-prepared' for the surge in registrations.
He said the electoral register has to be finalised and published six days before the referendum, noting time is needed to allow challenges against names on the register.
Brexit-backer Mr Jenkin warned Cabinet Office minister Matthew Hancock: 'That leaves very little time for anything like legislation.
'Can I advise you that it is probably legal to keep the site open for a short period, a few hours to capture those who did not have the opportunity to register yesterday.
'But any idea of rewriting the rules in any substantial way would be complete madness and make this country look like an absolute shambles in the run-up to this referendum, which is such an important decision.
'Will you bear these things in mind or risk judicial review of the result?'
Mr Hancock insisted the Government prepared 'extensively' for a peak in registrations, adding: 'The extent of the interest in registering was unprecedented.
'On the point you make about the length of period for which registrations may be valid in future in any legislative measure we bring forward, you suggest it should be for a short period and I agree.
'This is to rectify the problem of people not being able to vote last night and so we will bring forward proposals, most likely with that short period in mind.'
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