The EU is to send a "fact finding mission" to the Gibraltar-Spain border to investigate checks imposed on the Spanish side amid continuing tension between the UK and Spain over the issue. The warship HMS Westminster has arrived in the area.
The Foreign Office has voiced concern over comments made by Spain's foreign minister about the British territory of Gibraltar and says the UK will "not compromise" on British sovereignty over the area.
Spain is considering closing the airspace to planes flying to the British territory amid a growing dispute over fishing grounds.
Spanish Foreign Minister Jose Manuel Garcia-Margallo says Spain may introduce a 50-euro border crossing fee and is considering tax investigations of thousands of Gibraltarians who own property in Spain.
A border fee would affect tourists and Gibraltarians who cross the border for work.
It may be a pre-planned training exercise but what timing: UK warships sail through disputed water to Her Majesty's naval base on Gibraltar.
Spain could damage to the economy in Gibraltar, but in doing so it would harm its own, so a further escalation of threats is unlikely.
The PM has said he is seriously concerned by Spain's plans to introduce a levy at the border with Gibraltar.