Intensive checks on the Gibraltar border have been eased after William Hague made a direct appeal to his Spanish counterpart.
The Foreign Secretary telephoned Jose Manuel Garcia-Margallo to express "serious concern" after vehicle searches were stepped up over the weekend, causing massive delays in searing heat.
The British territory's government accused Spain of "torturing" vulnerable passengers and causing "unnecessary" hold-ups.
Mr Hague initially used diplomatic channels to raise the issue, but on Sunday night intervened directly.
A Foreign Office spokesman said the searches appeared to have been scaled down in the wake of the conversation.
The episode comes amid heightened tensions following a number of alleged Spanish incursions into British waters.
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."