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.
A Royal Navy warship is expected to arrive in Gibraltar later after more than 40 Spanish boats were involved in a confrontation with the British authorities.
The arrival of HMS Westminster, a type 23 frigate, is not part of Britain's response to the growing row.
The vessel left Portsmouth naval base in Hampshire six days ago to join nine other vessels taking part in a pre-planned international training exercise called Cougar '13 in the Mediterranean and Gulf.
Cougar '13 is a long-planned deployment involving four Royal Navy warships, the lead commando group from 3 Commando Brigade Royal Marines and elements of naval air squadrons.
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.