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.
Gibraltar's chief minister has accused Spain of acting like North Korea and sabre-rattling over the country's new hard-line stance on Gibraltar.
Fabian Picardo said Spanish foreign minister Jose Garcia-Margallo was being belligerent when he suggested that a 50 euro (£43.40) fee could be imposed on vehicles entering or leaving its border with the British Mediterranean outpost.
He told the BBC Radio 4 Today programme: "What we have seen this weekend is sabre-rattling of the sort that we haven't seen for some time.
"The things that Mr Garcia-Margallo has said are more reminiscent of the type of statement you'd hear from North Korea than from an EU partner."
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.