THE deadline for registering to vote in this month’s EU referendum has been extended until midnight tonight after the government's website crashed hours ahead of the initial cut-off on Tuesday.

Seemingly half a million Brits had left registering to vote until the last possible day, causing the website to become overwhelmed and unusable.

Voter registration is governed by exceedingly complex and quite unshiftable election laws. The extension requires emergency legislation which will be debated by MPs today.

Most parties have indicated their support, but some Eurosceptics believe the government is behaving in an underhand way, and that the sheer number of people using the website before the deadline was suspect.

The voters who have left it until the last minute to register are thought to be younger and more inclined to vote to Remain.

Matthew Elliot, the Chief Executive of Vote Leave, said it was clear “the government and their allies are trying to register as many likely Remain voters as possible.”

One senior Tory MP, speaking anonymously to the BBC, said: “We all know what they are up to - there will be a big social media push by Remain to get young people to register tomorrow.”

According to data provided by the government, 525,000 people applied to register to vote during the day - 170,000 were aged 25 to 34, 132,000 under the age of 25 and 100,000 aged 35 to 44.

The most users online at one time came at 10.15pm when 50,711 people were using the service.

Another Tory MP said: “They wouldn’t have done this at a general election” and another claimed: “If the age profile was the other way round we wouldn’t be doing this.”

Earlier in the day, one senior leave campaigner said extending the deadline could lead to the result of the referendum being subject to a judicial review.

Bernard Jenkin, the chair of the Commons Public Administration committee, said Cameron could be operating on the “cusp of legality”.

Jenkin told the BBC: “We are on the cusp of legality here. We are now outside the legal framework by keeping the registration open today,” he told BBC Radio 4’s The World At One.

“If they were to extend the registration for a period longer than this, they are really opening the possibility of a judicial review of the result if it is very close and people feel they have got a cause.

“We are not a banana republic. We shouldn’t be making up the rules for our elections as we go along. This is a shambles.”

Cameron told MPs during yesterday’s Prime Minister’s Questions: “I’m very clear that people should continue to register today.

“The Electoral Commission have made a statement this morning, urging the government to consider options that would effectively extend the deadline. These should include legislative options.

“We are doing that and discussing it with the Electoral Commission today.

“We are working urgently with them to do just that, to make sure that those who register today and those who registered last night will be able to vote in the EU referendum.”

SNP MP Owen Thompson said the whole mess proved that it was time for automatic registration: “In many democracies around the world, automatic registration is used to avoid a last minute administrative collapse.

“Voter engagement is vital for democracy to flourish – and we know from the way people got involved with the Scottish independence referendum that a high turnout and a high voter registration is incredibly energising to political debate and participation.

‘‘I am urging the UK government not to drag its feet any more on automatic registration – there cannot be any valid reason for them not to look seriously at it after this current fiasco.’’

LibDem leader and pro-Remain campaigner Tim Farron described the website problems as ‘‘a shambles’’ and said it could even affect the referendum result.

Meanwhile, the Electoral Commission have told returning officers to expect Scottish Referendum levels of turnout, and to be prepared for at least 80 per cent of all those who can vote to do so.