Those calling for a second EU referendum are also calling for the voting franchise to be extended to 16 and 17 year olds, resident EU nationals and all UK citizens resident overseas. This is a cynical move intended to influence the result towards "remain", should a second vote be held.
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Voting in UK national referenda must continue to be restricted to those eligible to vote in a General Election, namely those who are:
Registered to vote, aged 18 or over on the day of the election (‘polling day’), British, Irish or qualifying Commonwealth citizens, resident at an address in the UK (or a British citizen living abroad who has been registered to vote in the UK in the last 15 years) and not legally excluded from voting.