Commonwealth citizens based in Gibraltar are in danger of losing their right to vote in EU elections.
Gibraltar had its first European Parliament vote in 2004
The call by the advocate general of the Euro Court of Justice to partly uphold a complaint by Spain might also impact on UK-based Commonwealth citizens.
He said non-EU citizens in Gibraltar had no fundamental right to vote in European polls.
In 2003, the UK passed a law to include Gibraltar in the European constituency of the South West.
The advice is a preliminary opinion and is not binding on the European Court of Justice, which will make its final ruling in several months' time. However, the Court follows the advocate general's opinion in around 80 per cent of cases.
The UK has held Gibraltar since 1704. Spain ceded sovereignty in 1713 but has repeated claims to the territory, which is at its southern tip.
Labour MEP for the South West Glyn Ford said the plans would affect 100 Commonwealth citizens in Gibraltar and urged a rethink.
"This opinion goes against over 100 years of British history. Britain elected its first Commonwealth citizen, an Indian national, in 1883.
"In 2006 in British Gibraltar, he wouldn't be allowed to stand for election according to the European Court of Justice.
"Britain has a unique and important relationship with Commonwealth citizens. Any such ruling from the Court of Justice should be revised before it causes damage."
Last year, Graham Watson, Liberal Democrat MEP for the South West region, said the Spanish case owed more to "political spite" than rational argument.
He said: "Commonwealth nationals have always voted in European elections in the United Kingdom. All that happened in 2003 is that the same UK franchise was extended to Gibraltar.
"It is curious that Madrid has not raised this complaint before given that they clearly feel so strongly about it now."
Mr Watson said Spain had blocked changes to the European elections law. He argued the voting rights were logical because Gibraltar had been EU territory since 1973.
Neil Parish, Conservative MEP for the South West, has said the case is sad and politically motivated.
A spokeswoman from the Electoral Commission told the BBC News website that Irish and Commonwealth citizens are entitled to vote as British citizens, when resident in the UK.