LONDON (REUTERS) - British Prime Minister David Cameron has spoken to his Spanish counterpart Mariano Rajoy to raise "serious concerns" about tensions at the border with Gibraltar, his office said on Wednesday.

Mr Cameron spoke to Mr Rajoy by phone after Spanish Foreign Minister Jose Manuel Garcia-Margallo said in a weekend newspaper interview that Spain may introduce fees at the border with the British overseas territory and close its airspace to planes using its airport.

Spain disputes Britain's three centuries of sovereignty over Gibraltar, a territory on the southern tip of Spain that is home to close to 30,000 people with an economy dominated by offshore banking, Internet gambling firms and tourism.

Long-running tensions over the territory flared up last month when boats from Gibraltar dumped concrete blocks into the sea to create an artificial reef for fish. Spain said the reef would block its fishing boats.