Anyone who has not registered to vote by midnight will not be able to vote on May 7.
Despite nearly 2 million people registering in the last five weeks, there are still as many as 7.5 million who are unregisterd.
According to the Electoral Commission, there have been more than 1.7 million applications to register to vote in the past five weeks, with almost 470,000 online applications coming from 16 to 24-year-olds.
The Daily Mirror splashed the issue across its front page, saying "Your country needs you".
Celebrities and politicians are among those who tweeted an 11th-hour attempt to get people signed up.
Satirist Armando Iannucci, who created the political sitcom The Thick of It, held a Twitter forum yesterday to answer questions about why people should vote.
Ralf Little posted on his account: "Feel disillusioned? Even if you only vote to keep out the person you hate most, it all counts."
Deadline to register closes midnight tonight. Register here, takes 5 minutes. https://t.co/DOcvPLptNa. Closest election in decades.— Armando Iannucci (@Aiannucci) April 20, 2015
Green Party leader Natalie Bennett and shadow work and pensions secretary Rachel Reeves also reminded their Twitter followers to register in time.
Jenny Watson, Electoral Commission chairwoman, said: "This is your last chance to register to vote if you want to make your voice heard on 7 May. It takes just a few minutes to apply to register online, so do it now.
"We don't want anyone to miss out, but if you miss the deadline on 20 April and then try to vote you will be turned away from the polling station on election day. Make sure this doesn't happen to you."
How to register to vote:
The loss of a lot of voters has been blamed on a change that made people register individually instead of by household. Universities also lost the power to block register their students.
He said: "Having broken their promises on tuition fees to young people, having failed to build the economy that will work for them, having short-changed their future, this is David Cameron and Nick Clegg's final insult to young people.
"They are sitting by and watching hundreds of thousands of young people in our country lose their sacred democratic rights. We will not allow this scandal to happen and no right-thinking person should either."
Lib Dem MP Tom Brake replied: ""Instead of scaremongering, Labour should be working with their own local authorities to ensure that the large amount of money available is spent helping people, particularly students, register to vote."
The case for:
Hannah Vincent launched the campaign #XXVote to encourage young people - particularly young women - who are disengaged with politics to vote.
According to the campaign, only 51% of young people aged 18-25 voted in the 2010 general election and less than half of those were women.
Vincent said: "I was disengaged with politics. It was really confusing and uninteresting to me. We found out again and again that young people were overwhelmed by the amount of information you have to process as a first-time voter.
"It wasn't until I hit 26 that I realised the impact of a single vote.
"If you hold the power to decide who wins the General Election, they have to win you over. You need to be part of the process. You can't expect change if you're not part of the system.
"Even if you're sick to death by what politicians say or how the country is run, you don't have a leg to stand on if you're not actively voting, or at least registered to vote."
Suggest a correctionThe case against:
Comedian Russell Brand has been an outspoken critic of Britain's political system and claims to have never voted.
In an article for the New Statesman, he wrote: "I have never voted. Like most people I am utterly disenchanted by politics.
"Like most people I regard politicians as frauds and liars and the current political system as nothing more than a bureaucratic means for furthering the augmentation and advantages of economic elites."
In a Newsnight interview with Jeremy Paxman in 2013, Brand said voting is "tacit complicity" with a failing political system.
He said: "It's not that I'm not voting out of apathy, it's that I'm not voting out of absolute indifference, weariness and exhaustion from the lies, treachery and deceit of the political class that's been going for generations now."
Last month, at the launch of a new cafe staffed by recovering addicts in east London, Brand said: "As long as you have only got parties that are interested in causing division, hatred and representing big businesses, we will create our own systems.
''Politics is dead, this is the end of politics."