Spanish fishermen have mounted a protest against an artificial reef being built off Gibraltar that they claim has reduced their catch, and which has sparked a diplomatic spat between London and Madrid.

About 20 Spanish vessels set off on Sunday morning from Campo de Gibraltar in southern Spain, on the fringes of the British territory, and were met by 12 Gibraltarian law enforcement vessels, including the Royal Navy's HMS Sabre.

Gibraltar claims that the reef of concrete blocks is necessary to encourage the growth of fish stock and marine wildlife, but the Spanish claim it will affect their ability to fish in the waters, as their nets will become snagged in the blocks.

Gibraltar has been a British Overseas Territory since 1713, but the Spanish government disputes the UK's right to the land and surrounding waters, and the Spanish government has accused Gibraltar of creating the reef "without the necessary authorisation" in "waters that are not theirs".

Recently, Spanish authorities introduced stricter border controls on people entering and leaving Gibraltar, which they claim is to combat tobacco smuggling, and which have caused queues hours long.

UK Prime Minister David Cameron has protested to EU commission president Jose Manuel Barroso, calling the border checks "politically motivated and disproportionate" and claiming they contravene the EU right to free movement.

On Saturday Ukip MEP William Dartmouth, member for the south-west of England and Gibraltar, suggested that a member of the royal family should visit Gibraltar to mark 300 years of British sovereignty. "Nothing could demonstrate more to the citizens of Gibraltar how strongly Britain stands behind them," he told the BBC.

On Monday, Royal Navy warship HMS Westminster is due to arrive in Gibraltar after leaving Portsmouth last weekend, as part of a visit described by the Ministry of Defence as "long-planned" as part of "a range of regular and routine deployments".