The deadline for registering to vote in the EU referendum expires at midnight on Thursday following a temporary extension because of "technical issues" with the government website.
The registration website crashed on Tuesday night, hours before the initial deadline to take part, due to a surge in demand.
The Leader of the House of Commons, William Hague, has said "we should be concerned" if people haven't been able to register in time to vote for the EU referendum.
The online registration process closed at midnight on Monday, but many people could miss out on voting on June 23 after the site crashed due to last-minute demand.
Speaking to ITV News, Mr Hague said "a lot of attention" should be given to the issue and "everything possible" should be done to cater for those who missed out.
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.