Monday is the last possible day to register to vote in the 2015 general election.
Anyone wishing to vote must be registered by midnight on Monday or they will not be able to.
The Electoral Commission reports that 1.7 million applications to register to vote have been made in the past five weeks, most of them online.
People can register online or visit their local town hall offices to fill in a paper form.
It is too late to post a paper registration form.
The Electoral Commission said recent applications included almost 470,000 online applications from 16 to 24-year-olds.
However, it says there may be as many as 7.5 million unregistered voters among the 45 million people eligible to vote.
More than 120,000 people applied to register online on 19 April, the most in a single day so far. There were 110,000 applications on 16 April, the day the BBC aired the debate with opposition leaders.
The majority of those website visits were made after host David Dimbleby mentioned registration at the close of the programme.
The day with the next highest number of online applications was 30 March, which was the start of the official general election campaign period and the dissolution of Parliament.
Electoral Commission chairwoman Jenny Watson said: “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.”
Politicians including Nigel Farage and Natalie Bennett have been reminding their Twitter followers to register in time.
Ed Miliband tweeted: “Your vote on 7 May can help change Britain. But you need to make sure you’re registered before midnight tonight.”
‘Don’t miss out’
Among others to urge people to sign up include actor Michael Sheen, satirist Armando Iannucci and comedian and Labour activist Eddie Izzard.
Actor Ralf Little tweeted: “Feel disillusioned? Even if you only vote to keep out the person you hate most, it all counts. Don’t miss out.”
Campaign groups have been working to persuade ethnic minorities, disabled people and young people to register.
Rachael Harrington, from Voting Counts, said too often young people did not see the link between party politics and issues affecting their lives and even those who “did not agree with the system” should register and cast a vote.
“Go and speak to your local candidates,” she urged.
“If you decide you don’t agree with them, just go and spoil your ballot or blank vote. Just staying at home makes you another statistic of people who don’t care.”
Professor Ed Fieldhouse from the University of Manchester, told BBC Radio 5Live Breakfast that young people had traditionally been among the hardest to reach, with the older and better off more likely to register.
“You do find there is a connection with social class,” he said.
“People who are unemployed or in disadvantaged economic positions are less likely to turn out to vote.”
The UK general election takes place on 7 May. Local government elections are taking place across England – excluding London – on the same day. A number of mayoral elections are also taking place.
The last general election in 2010 saw a voter turnout of 65.1% across the UK, an increase on the two previous elections in 2005 and 2001 when turnout was 61.4% and 59.4% respectively.
The 2001 election was the first time since World War Two that turnout had fallen below 70%.
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