Although they might have retired to the Costa del Sol, moved to Sydney in search of sunshine or been lured to Dubai by the promise of lavish salaries, three million “missing” Britons could hold the key to success or failure at the general election in 2015.

Fewer than 20,000 are still registered to vote in Britain, even though they are entitled to cast a ballot if they have lived in this country in the last 15 years.

A major drive will be launched this year to track the expats down and encourage them to add their names to the electoral register in time for the European elections next May and the 2015 general election.

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People living overseas could also qualify to vote in any potential referendum on whether to pull out of the European Union.

The overseas recruitment drive could prove hugely significant in a closely-fought election.

Forty-two MPs were elected with majorities of less than 1,000 in 2010; if a few dozen voters could be persuaded to cast their ballots from Benidorm or Bondi, that could make all the difference in Bolton West or Broxtowe.

The Electoral Commission is drawing up plans for an online campaign targeted at people using email accounts abroad which were initially set up in Britain.

Reminders to register will also be sent to people overseas who use online search terms showing an interest in British politics and advertisements will be placed on the BBC and news websites known to be regularly accessed by UK nationals. Advertisements will also be taken on radio stations popular with Britons and expat newspapers.

Estimates of numbers of Britons nationals abroad vary between 4.5m and 5.5m. If those who have lived overseas for more than 15 years or are aged under 18 are excluded, more than three million could still be entitled to vote.

The Commission's top target countries will be Spain and Australia, each of which is home to around one million expats, followed by France, where some 500,000 British nationals live.

It will also focus on the United States, Canada, Ireland, New Zealand, South Africa, Germany and the United Arab Emirates.

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The Conservative MP Geoffrey Clifton-Brown, who is active in the Conservatives Abroad organisation, said last night: “It is pretty poor that these people are disenfranchised. If three million people were disenfranchised for a particular reason in this country, there would be an awful row about it.”

The Commission has already opened talks with embassies and consulates in the ten countries with the aim of helping them to get a “use your vote” message to expatriate communities. It is also giving advice to the major political parties, which all run overseas operations, to identify potential voters.

Mr Clifton-Brown challenged the Commission to set itself the target of registering 100,000 overseas voters as a “way of really concentrating minds”.

It has refused to be drawn on targets, but a campaign ahead of the last general election in 2010 led to more than 40,000 overseas registration forms being downloaded.

The number could be substantially higher this time because UK nationals will be able to register online next summer following the launch of a new electoral registration system.

The moves are in addition to the annual campaign to urge members of the armed services who are stationed abroad, and their families, to register. The largest numbers of non-operational troops are in Germany, followed by Cyprus.

Conservatives have tended to assume they would have the most to gain from more expats being added to the electoral register and make vigorous efforts to stay in touch with overseas supporters.

Some Tories claim the Labour government deliberately neglected to encourage the Electoral Commission to reach out to overseas voters.

Overseas voting: The rules

British nationals abroad can vote if they have lived in Britain in the previous 15 years. They are not allowed to choose where they vote; it has to be in the constituency where they were last on the electoral roll.

They are entitled to vote in elections to Westminster and the European Parliament, as well as national plebiscites such as the 2011 referendum on changing the electoral system.

They may not vote in local elections or contests for devolved bodies such as the Greater London Authority, Scottish Parliament or Welsh Assembly.