William Hague has raised “serious concerns” with Spain following claims it deliberately engineered lengthy delays at the border with Gibraltar following a weekend of heightened tensions.
The Foreign Secretary has telephoned his Spanish counterpart, Garcia Margallo, after Spanish officials searched every vehicle leaving the British territory.
Gibraltar's government accused Spain of "torture" after thousands of cars were stopped causing six-hour traffic jams in searing heat.
Mr Hague used British diplomatic channels to make Britain's concerns of the dispute known over the weekend but moved to intervene directly on Sunday night.
A Foreign Office spokesman said: "We are aware and very concerned about events at the Spain/Gibraltar border.
"Over the weekend, our Ambassador in Madrid has raised our concerns with the Spanish Deputy Foreign Minister and we have registered our protest with the Spanish Ambassador in London.
"The Foreign Secretary has also called the Spanish Foreign Minister Garcia-Margallo this evening to express our serious concerns and to urge a speedy resolution to the problems at the border."
Sunday's stand-off is just the latest incident to raise tensions between the British and Spanish governments over Gibraltar, which is at the southern end of the Iberian peninsula and has been under British rule since 1713.
Spanish police opened fire on a jet-skier in British waters and Spanish air force jets roaring across the territory, reportedly.
The Gibraltar government said the delays had affected thousands of people, according to a statement reported by the BBC. "The Spanish Government has inflicted these unnecessary delays on the elderly, children and the infirm in up to 30 degrees of heat.
"This torture has resulted in an ambulance being deployed to treat people with medical conditions. On Friday, for instance, a Spanish man had to be taken to hospital with chest pains."