Spanish fishermen have been involved in a stand-off with British Royal Navy and police boats after a flotilla made an illegal incursion into British waters around Gibraltar.

Around 38 fishing boats and a small number of pleasure craft were "corralled" by Royal Gibraltar Police and military vessels after crossing from Spanish waters close to an artificial reef created by the government of the British Overseas Territory.

The reef is at the centre of a diplomatic row between the UK and Spain, which has seen Madrid introduce additional checks at the border in protest, leaving workers and tourists facing queues of up to five hours to get through.

Chief Inspector Castle Yates, of the Royal Gibraltar Police, said the boats met in Spanish waters and, despite efforts by British boats to stop them, crossed into Gibraltan waters before being "pushed" out again.

He said police had been aware of the planned protest since Friday and it had passed peacefully, with no arrests.

The floating protest was also met by Spanish Guardia Civil boats, which warned them not to sail too close to the British territory's reef.

The Spaniards, who set out from the Campo de Gibraltar in the country's south, claim the reef restricts their right to fish.

Spain believes Gibraltar's dropping of concrete blocks to the sea floor creating an artificial reef was done to disrupt their fishing fleet.

Gibraltar says it was necessary to protect local fish stocks.

A diplomatic spat between the UK and Spain erupted when the Spaniards introduced additional checks at the border, suggesting that a £43.30 (€50) fee could be imposed on every vehicle entering or leaving Gibraltar through its fenced border with Spain.

On Friday British Prime Minister David Cameron raised the imposition of the extra checks with European Commission president Jose Manuel Barroso.

In a telephone call, he underlined Britain's belief that the checks were "politically motivated and disproportionate" and therefore contrary to the EU right of free movement.

Fabian Picardo, Gibraltar's chief minister, thanked the UK authorities for their help today.

Mr Picardo, who has reportedly received death threats and been targeted by Spanish internet trolls, wrote on Twitter: "Big thank you also to Royal Navy, Gib Defence Police, HM Customs and Port Authority for their deployment too.

"Cool, professional and calm!".