Mariano Rajoy is understood to have promised to de-escalate situation after complaint from David Cameron
The Spanish prime minister has pledged to reduce delays at the border with Gibraltar after David Cameron personally intervened in the simmering diplomatic dispute.
Cameron rang his Spanish opposite number, Mariano Rajoy, to complain formally over the threat to impose fees on the border between Spain and Gibraltar.
The prime minister said in a message on Twitter: "Constructive call with Spain's PM Rajoy. I made clear my concerns re Gibraltar and that our position on sovereignty won't change."
Rajoy is understood to have pledged to Cameron that he would de-escalate the situation at the border, and try to end the border delays.
A No 10 spokeswoman said: "The prime minister called the Spanish prime minister, Mariano Rajoy, this morning to raise serious concerns about actions by the Spanish at the border with Gibraltar and suggestions from Spanish foreign minister, José García-Margallo, that they may introduce further measures.
"The PM made clear that our position on the sovereignty of Gibraltar and its surrounding waters will not change. He also reiterated, as the PM and Mr Rajoy had previously agreed, that the issue should not damage our bilateral relations. However, there was a real risk of this happening unless the situation at the border improved.
"Mr Rajoy agreed that he did not want the issue to become an obstacle in the bilateral relations and that we needed to find a way to de-escalate the issue. As a next step, the foreign secretary should speak to Mr García-Margallo to discuss a way forward.
"In the meantime, Prime Minister Rajoy committed to reducing measures at the border. Both leaders agreed that there should be a solution to the fishing dispute."
The prime minister returned to Britain from holiday on Monday and immediately dispatched Giles Paxman, the British ambassador to Madrid, to complain formally about the mounting threats to the island, including the possibility of closing air space.
But Cameron decided to intervene after UK ambassador failed to receive assurances that the Spanish would not impose a €50 border crossing charge, a move that would seriously threaten the livelihoods of hundreds of Gibraltarians who travel to Spain each day for work.
Cars travelling to Spain have been subject to extensive delays.
There had been hopes that the threats were limited to statements by the Spanish foreign minister, but it is understood work is now under way across the Spanish government about what punishments could be inflicted on Gibraltar. Some of the row has been stoked by the Spanish rightwing government's need to distract from corruption charges that have been levelled against its most senior figures.
Downing Street believes the Gibraltar dispute is not going to dissipate this month, and a direct complaint at prime ministerial level was appropriate.
The dispute, at root a centuries-old argument about sovereignty of the rock off Spain, has been sparked by the Gibraltar government dumping 70 concrete blocks in the bay, something that has angered Spanish fishermen.
Britain believes the potential move on border fees breaches bilateral agreements and may also fall outside EU rules. But proportionate fees may be possible since the UK is not part of the Schengen agreement.
