Residents of Gibraltar will be able to vote as part of England's South West region in the next Euro elections.
The change follows a human rights case by a Gibraltar woman
Election officials decided the Rock is too small to be designated as a European parliamentary region on its own and needed to be "attached" to a mainland region.
It means Gibraltarian parties can stand in the South West, with those MEPs elected charged with representing both that region and Gibraltar in the European Parliament.
Voters in Gibraltar, who number less than 20,000, will also have a say in who is elected to represent the region, which currently has about 5 million voters.
There was broad support among Gibraltar's political leaders for joining the South West, and all but one of the region's current MEPs actively promoted the addition of Gibraltar.
Sam Younger, chair of the Electoral Commission, said: "Our priority has always been to ensure the most effective representation of the people of Gibraltar.
"We believe that combination with the South West best satisfies this condition and will be broadly welcomed by people in Gibraltar and in the region."
The move follows a ruling by the European Court of Human Rights in 1999.
The court said the electoral rights of Denise Matthews, a British citizen living in Gibraltar, had been violated because she could not vote in European elections.
The commission considered a number of criteria in its investigation - including which region is closest to Gibraltar, and which region shares tourism, maritime and industry interests with the British colony.