Tension was running high Friday between Spain and Britain over an artificial reef at Gibraltar, with Madrid saying it reserved the right to take "legal and proportionate" measures to defend its interests, according to dpa. "Spain and the government will take the decisions that they deem timely and convenient," Prime Minister Mariano Rajoy said after discussing the dispute with King Juan Carlos in Palma de Majorca. British warships were meanwhile due to set sail for Gibraltar in what Spain and Britain described as a routine military exercise. The outcrop of 6.8 square kilometres on Spain's southernmost tip has long soured relations between London and Madrid, which claims sovereignty over Gibraltar, under British rule for three centuries. Tension rose again after Gibraltar placed 70 concrete blocks in the sea. It says the reef protects biodiversity. But Rajoy said the reef was an "attack against the environment" and disrupts the work of Spanish fishermen. The row has prompted Spain to tighten controls of vehicles at the Gibraltar border, causing queues lasting for hours. Madrid has also threatened other measures, including a 50-euro (65-dollar) border fee to access Gibraltar and stiffer tax controls on more than 6,000 Gibraltarians living in Spain. The two countries launched talks to solve their dispute, but little progress has been reported so far. Rajoy said Spain was now seeking four-partite talks with Britain, Gibraltar and the neighbouring southern Spanish region of Andalusia. Gibraltar rejects such talks, because they would place it on an equal footing with a Spanish region and reduce it to a junior negotiating partner. Gibraltar wants to relaunch the tripartite talks that were conducted under Spain's previous Socialist government. Those talks allowed Gibraltar to negotiate with Spain and Britain as an equal partner. Britain's Daily Telegraph meanwhile said a rapid reaction force of warships, led by the light aircraft carrier HMS Illustrious, will sail for the Mediterranean on Monday for a four-month deployment. Some of the ships, including a frigate, will stop at Gibraltar. The visit was announced two days after Gibraltar Chief Minister Fabian Picardo demanded that warships be sent to stop Spanish incursions into British territorial waters, Telegraph said. The Spanish government described the exercise as routine and said it had authorized the ships passing through Gibraltar to also moor at the naval base of Rota in southern Spain. Rajoy said the plans were known since June 4. British naval chiefs also said the fleet was part of a long-planned exercise, and not in response to the diplomatic row. Spanish officials meanwhile said a British European Parliament member had filmed Spanish police controlling vehicles on the border, accompanied by Gibraltar police. Spain had taken action to stop such activities, the officials said.