It comes after a survey of UK citizens living overseas carried out by the UK electoral commission showed that half of all respondents do not know that they can now register online to vote in the UK's EU referendum.
Alison Rose, Britain's top diplomat in Belgium, is now backing a call for expats to take part in the vote if they can.
There are an estimated 30,000 British nationals living and working Belgium, home to the EU institutions and organisations such as Nato.
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Rose said, "The UK electoral commission is recommending UK nationals overseas to register to vote by 16 May if they want to vote by post in the referendum. This allows enough time to receive and return a postal ballot."
She added, "They don't need to wait for registration to be confirmed before choosing whether to vote by post, by proxy (voting by appointing someone you trust to vote on your behalf), or even in person at your polling station if they will be in their local authority area on polling day."
"If someone registered last year to vote in the UK general election they need to renew their registration in order to vote in the referendum. Overseas voters have to renew their registration every year."
To register as an overseas voter, UK residents overseas must have previously been registered in a UK constituency within the last 15 years.
Anyone who is unsure if they are registered to vote are being told they should contact their last UK local authority to check their status.
Registering as an overseas elector makes them eligible to vote in UK parliamentary elections, UK-wide referendums and elections for the European Parliament.
British citizens in Belgium need to apply to vote by Monday and their national Insurance number and passport details are needed.
Once registered they should apply to vote by post or proxy in order to be able to vote from outside the UK.