The British Foreign Secretary William Hague has put pressure on Spain over claims it is deliberately engineering lengthy delays at the border with Gibraltar.
The Foreign Secretary telephoned his Spanish counterpart, Garcia Margallo, to raise "serious concerns" after a weekend of heightened tensions.
Spanish officials carried out intensive vehicle searches in searing heat causing massive hold-ups.
Thousands of cars leaving the British territory were stopped for checks causing six hour traffic jams.
Gibraltar's government accused Spain of "torture" by inflicting searches on vulnerable passengers and causing "unnecessary delays".
Mr Hague used British diplomatic channels to make Britain's concerns of the dispute known over the weekend but tonight moved to intervene directly.
A British Foreign Office spokesman said: "We are aware and very concerned about events at the Spain/Gibraltar border.
It comes after relations were strained following a number of accusations of Spanish incursions into British waters.
The Gibraltar government said the delays had affected thousands of people.