Labour MP Harriet Harman has taken to Twitter to ask why people need to register to vote.
More than 26,000 people were using the online service to register with just five minutes to go before the Monday deadline, allegedly causing the website to crash.
Harman said all eligible people should be put on the register automatically - instead of being asked to apply.
Why on earth do we still have "opt in" elec registration? U shouldn't have to apply. Govt should put all eligible on register
Leave campaigners argue the move is an attempt "to rig the referendum".
MPs demand extra time for people to register to vote in the EU referendum after 'unprecedented demand' caused the website to crash.
The government website has gone down as the deadline to register to take part in the EU referendum expires at 23:59.