The British government is considering taking legal action against Spain over stringent border checks imposed at the border with Gibraltar, a spokesman for Prime Minister David Cameron said Monday.
The spokesman said the checks by Spanish guards, which have caused tailbacks of several hours at the border of the British-held territory, were "politically motivated and totally disproportionate".
"Clearly the prime minister is disappointed by the failure of Spain to remove the additional border checks this weekend," the spokesman told reporters.
"We are now considering what legal action is open to us.
"This would be an unprecedented step so we want to consider it carefully before a making a decision to pursue."
Britain and Spain are embroiled in an increasingly tense diplomatic spat over Gibraltar, a tiny self-governing British territory at the southern tip of Spain.
Gibraltar has accused Madrid of imposing the checks in retaliation for its construction of an artificial concrete reef off its coast, which it says is aimed at stopping alleged incursions by Spanish fishing boats.
Madrid claims the border checks are necessary to combat smuggling and that the reef is a deliberate impediment to Spanish fishing vessels in a dispute over territorial waters.
A handful of British warships began setting sail for the Mediterranean on Monday on what the defence ministry stresses is a routine exercise that was planned months ago.
But one of the ships is set to dock in Gibraltar later this week in a move that is being seen by Spanish media as an act of intimidation.