MADRID (REUTERS) - Spanish Prime Minister Mariano Rajoy said on Friday that he hoped for dialogue with Britain soon regarding Gibraltar, but added that until talks took place, his government would continue to consider unilateral measures to defend Spanish interests.

Tensions over Gibraltar - a British overseas territory that Spain also has claims to - flared up last week when Spain complained that an artificial reef being built by Gibraltar would block its fishing vessels.

"I hope that this doesn't go any further, but it's clear that Spain has to defend its national interest and that's what we're going to do," Mr Rajoy said to reporters after a meeting with Spain's King Juan Carlos in Mallorca.

Spain says it is considering measures such as a 50-euro (S$84) border fee for people entering Gibraltar from Spain, tax investigations of Gibraltarians with property in Spain and restrictions on use of its airspace for flights going to Gibraltar's airport.

Mr Rajoy said he had a constructive phone conversation on Wednesday with Britain's Prime Minister David Cameron to try to bring a war of words over Gibraltar to an end.

Spanish Foreign Minister Jose Manuel Garcia-Margallo spoke with his British counterpart on Wednesday and said they agreed to set up working groups on issues such as fishing around the tiny territory known as "The Rock," at the tip of Spain.

However, the Gibraltarian authorities threw the diplomatic efforts into doubt, saying no working groups had been agreed.

Mr Rajoy said on Friday that any talks must be four-way, involving Spain, Britain, Gibraltar and Andalusia, the Spanish region that neighbours Gibraltar and is home to many workers who cross the border daily into the British territory.

Spain ceded Gibraltar to Great Britain in the Treaty of Utrecht, a document in Latin that was signed 300 years ago.