The Prime Minister has called on the European Commission President Jose Manuel Barroso to urgently send a team of monitors to Gibraltar's border with Spain to collect evidence of Spanish border checks he said broke EU law.
David Cameron's spokesman said: "We believe that the European Commission, as guardian of the treaties, should investigate the issue," adding that the extra Spanish checks were "politically motivated and disproportionate".
The spokesman added: "President Barroso responded that the European Commission are closely monitoring the situation and that, following a thorough legal assessment, they would not hesitate to take any measures necessary to uphold EU law."
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.