LONDON (AFP) - London mayor Boris Johnson on Monday issued a "hands off" warning to Spain over Gibraltar after Madrid vowed to take "all necessary measures" to defend its interests in the British outpost.

The gaffe-prone Conservative mayor urged the government to "prise Spanish hands off the throat of our colony" after it increased border checks in retaliation for the building of an artificial reef in disputed waters.

"Hands off our Rock, that's what I say," Mr Johnson wrote in Monday's Daily Telegraph.

"I hope that one way or another we will shortly prise Spanish hands off the throat of our colony, because what is now taking place is infamous," he added.

Britain last week said it was sending a force of warships to Gibraltar on its way to naval exercises in the Mediterranean, while Spanish Prime Minister Mariano Rajoy said his country would take "all necessary measures" to defend its interests.

Spain has mooted a 50-euro (S$84) toll to enter or leave Gibraltar, saying proceeds would be used to help fishermen who have suffered from the creation of an artificial reef by the British outpost.

Mr Johnson accused Spain of reviving the "general harassment of the Franco epoch".

He compared Spain's claims on Gibraltar to those of Argentina on the Falkland Islands, saying they were both being used to divert attention away from domestic problems.

"This isn't a row about fish," he said. "I am afraid that this is a blatant diversionary tactic by Madrid.

"The euro is the crisis facing the Spanish government, not the right of the Gibraltarians to fish off their own Rock."

Spain ceded Gibraltar to Britain in perpetuity in 1713 but has long argued that it should be returned to Spanish sovereignty. London says it will not do so against the wishes of Gibraltarians - who are staunchly pro-British.

The self-governing British overseas territory, measuring just 6.8 square km, is home to about 30,000 people, and is strategically important as it overlooks the only entrance to the Mediterranean Sea from the Atlantic Ocean.