The row between Britain and Spain over Gibraltar has intensified with London threatening legal action over "totally disproportionate" border checks.
The comments came as British warships set sail for the Mediterranean on Monday for a naval exercise, in which the HMS Westminster will dock in Gibraltar.
Helicopter carrier HMS Illustrious steamed out of Portsmouth, to be followed on Tuesday by the type-23 frigate HMS Westminster, which is set to arrive in Gibraltar within a week.
The defence ministry has stressed that the deployment of the ships is "routine" and "long planned".
But in a hardening of Britain's tone, a spokesman for Prime Minister David Cameron said the government was considering legal action over the checks by Spanish guards on the border of the rocky outpost on Spain's south coast.
The spokesman said the checks had caused delays of several hours, and 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 said.
"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."
Spain has refused to stop the checks, which it says are "legal and proportionate".
London's threat came after Spain said it was considering taking the row over the disputed territory to global bodies.
A foreign ministry spokesman said Madrid was "evaluating the possibility of going to bodies like United Nations, the security council, the court in The Hague".
But he said that "no decision has been taken" on the possible course of action.
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.
