The row between Britain and Spain over Gibraltar escalated as London said it was considering taking legal action over "totally disproportionate" border checks, imposed after Gibraltar created an artificial reef which Spain said blocked its fishing vessels.
A spokesman for British Prime Minister David Cameron said on Monday that the checks by Spanish guards were "politically motivated and totally disproportionate".
The tougher checks at the 1.2km border of the rocky outpost on Spain's south coast have caused long delays for thousands of tourists and local people.
"Clearly the prime minister is disappointed by the failure of Spain to remove the additional border checks this weekend," the spokesman said, adding that the UK is considering what legal action is open to them.
Spain said it would not back down over the border controls which it said were a legal and proportionate step to prevent money laundering and smuggling of tobacco and other products from Gibraltar.
A Spanish foreign ministry spokeswoman on Monday restated her country's position that it was considering what international forum it could use to press its claim to Gibraltar.
Gibraltarians were granted full British citizenship in 1981 and a referendum in 2002 backed Britain's rule, with 98 percent of voters rejecting the idea of shared sovereignty with Spain.
The self-governing British overseas territory, measuring just 6.8 square kilometres, is home to about 30,000 people and is strategically important as it overlooks the only entrance to the Mediterranean from the Atlantic Ocean.