British helicopter carrier HMS Illustrious has sailed out of Portsmouth harbour in southern Britain to join a British naval task force heading to Gibraltar.

The timing of the deployment has been described by Britain's Ministry of Defence as long-scheduled and coincidental, but comes amid rising tensions between London and Madrid over the British overseas territory.

Gibraltar, the tiny rocky promontory near the southern tip of the Iberian Peninsula, has been a source of on-off tensions since Spain ceded the territory to Britain in the Treaty of Utrecht 300 years ago.

The latest dispute arose last month when Gibraltar's boats dumped concrete blocks into the sea to create a reef for fish at the mouth of the Mediterranean.

Spain said the reef would restrict its fishing boats and hit back with tougher border checks and threats of a 50-euro fee for people crossing the Gibraltar border. It is not clear whether such a fee would be legal under EU law.

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