Britons living abroad are being urged by the Electoral Commission to register to vote in the UK's EU referendum before the 7 June midnight deadline.
Some expats have told the BBC there has been confusion over the registration cut off, which some thought was 16 May.
This was the advisory date widely publicised to make sure people had time to get and return a postal vote, the Electoral Commission said.
Only those on the UK electoral register in the last 15 years can register.
The in-out referendum on the UK's membership of the European Union takes place on 23 June.
Midnight on 7 June is the final registration deadline for both UK and overseas voters.
The deadline to apply for a postal vote is 17:00 BST on 8 June. Postal vote ballot papers must arrive back by 22:00 BST on 23 June to be counted.
Overseas voters who want to vote by proxy must register and then complete and return an application form to their local electoral registration office by 17:00 BST on 15 June in Great Britain, and by 17:00 BST on 3 June in Northern Ireland.
Who can vote in the referendum?
You can vote in this referendum if you are registered to vote in the UK, are 18 or over on the day and are:
- A British or Irish citizen living in the UK
- A Commonwealth citizen living in the UK who has leave to remain in the UK or who does not require leave to remain in the UK
- A British citizen living overseas who has been registered to vote in the UK in the last 15 years (For those who were too young to register when they left the UK, their parent or guardian must been have registered)
- An Irish citizen living overseas who was born in Northern Ireland and who has been registered to vote in Northern Ireland in the last 15 years
- A citizen of Gibraltar
Those eligible can register to vote online.
More information about postal votes or using a proxy voter is available at www.aboutmyvote.co.uk.
Daniel Tetlow, a British expat living in Berlin who is part of a campaign to encourage Britons abroad to sign up to vote, told the BBC there had been "misinformation" about the cut off point, as the 16 May advisory date had been misinterpreted in some quarters as the final deadline.
For example, a tweet from the British embassy in Berlin stated the 16 May was "the deadline" to register as an overseas voter in the EU referendum.
The confusion was "obviously serious and threatens to disenfranchise thousands of Brits who have the right to vote, but don't know it", he said.
Jane Golding, also a member of the campaign, said there had been a spike in applications to register by British citizens abroad on 15 and 16 May - prior to the advisory deadline - but numbers had since dropped off.
The Electoral Commission had done "great work" with its registration drive, but the potential misunderstandings were "concerning", she said.
With just one week to go before registration closes, the Electoral Commission says many people are still not signed up to vote in the EU referendum.
Campaign groups Hope Not Hate and Bite The Ballot are launching a week of action on Wednesday to encourage 500,000 young people to get their name on the electoral register.
Bite The Ballot has also teamed up with dating app Tinder to launch Swipe The Vote UK, which aims to encourage 18 to 34-year-olds to cast a vote on 23 June.