"Gibraltar is a colony of the British Crown,” the text said, according to the Financial Times. “There is a controversy between Spain and the UK concerning the sovereignty over Gibraltar, a territory for which a solution has to be reached in light of the relevant resolutions and decisions of the General Assembly of the United Nations.”

It comes as Britain continues to hurtle towards its 29 March exit date from the EU without any agreement in Parliament over Theresa May's withdrawal deal.

With the prospect of a no-deal Brexit looming, many lawmakers are calling for an extension of Article 50, which would give the prime minister more time to negotiate a deal acceptable to MPs.

A senior EU diplomat told the newspaper: “The Spanish are gearing up for a Gibraltar fight when there is an extension request. It could be dangerous.”

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The row comes just weeks after a Spanish warship sailed past the territory while apparently playing the Spanish national anthem over loudspeakers.

The Spanish government claims the Iberian territory should be returned to Spain, a policy adopted during the Franco era. Polls and referenda results however show Gibraltans overwhelmingly wants to remain part of the UK.

Last month, defence secretary Gavin Williamson said Gibraltar would "always be under the Union flag”.