Tensions between London and Madrid over Gibraltar are growing after the Spanish Prime Minister rebuffed an appeal by David Cameron to scale back contentious border checks with the British territory.
Mr Cameron called his counterpart, Mariano Rajoy, to spell out his "serious concerns" after the Spanish government raised the prospect of imposing a €50 (£43) on vehicles entering Gibraltar and closing Spanish airspace to flights heading for the Rock.
The diplomatic crisis - which blew up over a dispute over fishing waters - was brought to a head at the weekend as drivers were forced to queue for up to six hours before they were allowed to cross into Gibraltar.
Britain has already registered a formal complaint to the Madrid and is understood to be contemplating legal action to get the moves blocked.
Mr Cameron warned his counterpart in a 15-minute phone call of the "real risk" of harm to relations between the countries unless the problems over frontiers checks were resolved.
Downing Street said he had obtained a promise from Mr Rajoy to reduce the protracted checks at the border.
However, a statement from the Spanish Prime Minister's office painted a different picture. It made no reference to cutting border checks and reported he had told Mr Cameron "the adopted measures were perfectly in accordance with the Schengen border code", which governs travel between EU member states.
British officials acknowledged there was still much work ahead to "de-escalate" the stand-off.
Nor was there any prospect of Brussels brokering a rapid solution to the stand-off. The European Commission is sending a team to examine the border controls, but does not expect talks to take place with Spain on the issue until next month at the earliest.
It confirmed the controls are allowed because Gibraltar is not part of the Schengen agreement, which permits people to travel freely within the Schengen area of 26 countries. But it stressed checks had to be proportionate.
Speaking after the call, Mr Cameron said the delays faced by Gibraltarians were unacceptable and said Britain would always stand up for them.
"Of course there is a fishing dispute between Gibraltar and Spain and that needs to be settled and we agreed that should be settled but it is not right to escalate things in the way that has been done and I made that very clear to the Spanish prime minister.
"We agreed that our foreign ministers would speak and try to resolve these issues," he said.
Giles Paxman, the UK's ambassador to Spain, has delivered a formal protest over the moves to Gonzalo de Benito, the Spanish foreign secretary.
Jose Garcia-Margallo, a foreign minister, said the proceeds of a border fee could be used to help Spanish fishermen who have lost out because of damage to fishing grounds allegedly caused by Gibraltarian authorities following the creation of an artificial reef.
William Hague, the Foreign Secretary, spoke to Mr Garcia-Margallo to reinforce Britain's message the continuing delays were "disproportionate". He said: "I urged the Spanish authorities to de-escalate the situation."