The diplomatic row between Britain and Spain over extra checks at the border with Gibraltar has intensified after the UK government said it was considering taking legal action.
Downing Street said it was looking at the “unprecedented step” after the Spanish government failed to lift the additional controls over the weekend.
Spain later said it would not abandon the checks, saying they were “legal and proportionate”.
The European Commission plans to send a team of investigators to the border next month, who will observe the controls, following complaints from several MEPs and EU citizens about long waits there.
It comes as three Royal Navy warships set sail for the Mediterranean in what defence officials stressed was a long-scheduled deployment.
The vessels included the flagship HMS Bulwark, helicopter carrier HMS Illustrious and frigate HMS Montrose.
As part of the operation, a fourth warship, the frigate HMS Westminster, will dock in the UK overseas territory of Gibraltar for a routine visit for three days this month, while other ships in the task group are expected to go to Spanish ports.
A No 10 spokesman said: “Clearly the prime minister is disappointed by the failure of Spain to remove the additional border checks this weekend. We are now considering what legal action is open to us.”
The diplomatic spat between the UK and Spain was sparked by the creation of an artificial reef by the Gibraltarian authorities, which the Spanish claim will destroy fishing in the area.
Madrid responded by beefing up border controls, leading to lengthy queues, and suggesting that a 50 euro (£43.30) fee could be imposed on every vehicle entering or leaving the Rock through the fenced border with Spain.
The Royal Gibraltar Police tweeted on Sunday that Guardia Civil checks had caused queues of up to two hours at the border, with Spanish officers checking “every car” going into the Rock.
Reports in the Spanish media suggested that the diplomatic row could escalate to the United Nations, with Prime Minister Mariano Rajoy’s government receiving support from Argentina.
Spanish foreign minister Jose Garcia-Margallo is expected to propose that the two countries present a “united front” over Gibraltar and the Falkland Islands, the El Pais newspaper reported.
Mr Garcia-Margallo will sound out his Argentinian counterpart, Hector Timerman, during a meeting in Buenos Aires next month as he prepares for a “180-degree turn in policy towards the colony”, the newspaper said.
Meanwhile, Spanish defence minister Pedro Morenes has said it is “totally normal” for British warships to dock in Gibraltar in what is a “routine visit”, according to reports.
Mr Morenes reportedly told the Europa Press news agency: “Neither the British nor the Spanish government have an interest in there being bad relations.”
He also said that aircraft carrier HMS Illustrious had requested and been granted permission to stop off at the southern Spanish naval base of Rota on August 18, proving that there was no military escalation between the two countries.
The MOD’s operation in the Mediterranean, Adriatic and Gulf – codenamed Cougar 13 – is due to last around four months and will include a series of exercises as well as escort duties and counter-piracy operations.
Four Royal Navy warships, the lead commando group from 3 Commando Brigade Royal Marines and elements of naval air squadrons will be supported by five vessels from the Royal Fleet Auxiliary.