The Foreign Secretary William Hague has put pressure on Spain over claims it is deliberately engineering lengthy delays at the border with Gibraltar.
In a telephone call with his Spanish counterpart, Garcia Margallo, Mr Hague raised "serious concerns" over severe delays caused when officials carried out intensive vehicle searches.
Thousands of cars leaving the British territory were stopped for checks causing six hour traffic jams in the searing heat.
Gibraltar's government accused Spain of "torture" by inflicting searches on vulnerable passengers and causing "unnecessary delays".
Mr Hague has used British diplomatic channels to make Britain's concerns of the dispute known over the weekend but tonight moved to intervene directly.
More top news
Dan Rivers explores the island community made up of descendants from Bounty mutineers, with a story worthy of a Hollywood sequel.
The US president raised laughs as he read out a series of insults about himself - but it ended with a tweet from Trump himself.
Experts have warned pregnant woman to avoid non-essential travel there.