Visserijdispuut tussen Spanje en Gibraltar (en)

BRUSSELS - Spain has said it is considering imposing a €50 fee on those entering and leaving Gibraltar amid an escalating row with the UK over fishing rights.

Foreign Minister Jose Manuel Garcia-Margallo told ABC Sunday (4 August) that such a vehicle levy could "help fishermen affected by the destruction of fishing grounds."

He also announced a series of other potential measures such as sending the Spanish tax authorities to investigate the around 6000 Gibraltarians with property in Spain, closing Spanish air space to flights heading to the rocky outcrop and making its online gaming companies operate under Spanish law.

The UK reacted to the statements about its overseas territory by saying it "will not compromise on sovereignty."

"We will be seeking an explanation from Spain following reports that the Spanish government might target Gibraltar with further measures," said the foreign ministry, urging no "disproportionate" measures.

The origins of the row began around two weeks ago when the Gibraltar authorities started putting concrete in the sea around the territory saying an artificial reef would boost fish stocks.

Madrid, which disputes Britain's 300-year sovereignty over the small territory on the southern tip of Spain, argued the move is designed to block Spanish fishermen from the waters.

It lodged a complaint with the UK saying the planned reef blocks "Spanish fishermen fishing in a manner that is contrary to our law."

Soon after the work on the reef start, Spanish authorities stepped up border checks at the Spain-Gibraltar border.

The move caused hours-long delays although the authorities denied that the checks were in retaliation for the reef.

Gibraltar, for its part, complained to the European Commission about the border checks.

The territory, known as The Rock and ceded to the UK under the Treaty of Utrecht in 1713, has regularly been a source of tension between Madrid and London.

The previous socialist government in Spain took a softer stance on the issue of Gibraltar - home to about 30,000 people - but the current centre-right government under Mariano Rajoy i is seen as taking a harder line.