Labour leader Jeremy Corbyn and the Liberal Democrat's Tim Farron have demanded extra time for people to register to vote in the EU referendum after "unprecedented demand" caused the government website to crash.
A last-minute rush to sign up was stalled after the website failed, with Mr Corbyn and Mr Farron among those demanding an extension.
Mr Farron said it could be a "major blow" to the prospects of the UK remaining in the EU if young people were denied a say.
This is a shambles the Government has presided over and people must be given an extra day to exercise their democratic right. It is also a major blow to the 'In' campaign and our prospects of staying in Europe With individual voter registration, and a big campaign to encourage young people to register, many of whom have been trying to do so last minute, this could have major consequences for the result. Evidence shows younger people are overwhelmingly pro-European, and if they are disenfranchised it could cost us our place in Europe. It could also turn them off democracy for life. Voters must be given an extra day while this mess is sorted out urgently.
The Electoral Commission - which oversees the running of the referendum - tweeted at 12:01am that anyone who had failed to register by the deadline at 23:59 would not be able to take part in the referendum.