The expat vote is expected to have a decisive impact on the referendum on whether Britain should remain within the EU with polls currently showing it is neck and neck.
But with those living abroad the most likely to be against a Brexit, could they tip the balance?
Only those who have been on the electoral roll in the UK within the last 15 years are eligible to vote under the same rules that apply to general elections.
The so-called '15-year rule' prevents around a million of the estimated five million Britons living overseas from voting in UK parliamentary elections.
But the British Embassy in Spain has launched a campaign to encourage those expat residents who can to register.
Of the 283,000 Britons registered on the padrón as resident in Spain, just 11,000 are already registered in the UK.
British Ambassador Simon Manley has a message for expats. Photo: FCO
"Interest among expats in the EU referendum is high, but awareness that you may be able to vote is low. We want as many expats as possible to be aware that they can have their say," said Simon Manley, British Ambassador to Spain, at a meeting with expats in Javea.
"You can register to vote in a few minutes via the government website - and do encourage your friends, family and colleagues to do the same," he said.
The campaign emphasizes that registering is so simple and takes less than five minutes but should be done well ahead of the deadline to allow time for a ballot paper to be posted from the UK and return as a postal vote.
Although no date has yet been set, the referendum looks likely to be held as early as June 2016 although it could be put back to the autumn if David Cameron does not reach a deal before the February 19th meeting of EU leaders.
The question, which has already been approved in the EU referendum bill will ask "Should the United Kingdom remain a member of the European Union or leave the European Union?"