Spanish Prime Minister Mariano Rajoy has offered to " reduce measures " at the Gibraltar border following a phone call with David Cameron, Number 10 has said. Earlier, the prime minister said he had conducted a " constructive " conversation with Mr Rajoy. The call followed a formal protest from Britain ' s ambassador to Spain over the " disproportionate " border checks. Mr Rajoy repeated during the exchange that Gibraltar ' s creation of an artificial reef was " unacceptable ". Mr Cameron called to express his " serious concerns " over deteriorating relations, Number 10 ' s press office said. Last weekend increased vehicle checks at the border caused major delays for tourists and locals. The rigorous checks followed Gibraltar ' s move to drop concrete blocks into the bay to start creating an artificial reef which it said would encourage sea life. Spain also said it was considering a new 50 euro(£43) fee to cross the border with the British territory. After making a bid to ease tensions over the telephone, Mr Cameron said 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 ". Mr Rajoy said during the 10 - minute conversation that the relations between the two countries and their disagreements should be resolved with " honesty ", the Spanish news agency EFE said. The Spanish leader also maintained that " the unilateral action of installing concrete blocks in the Bay of Algeciras was unacceptable ".Risk to relations Number 10 said the prime minister also reiterated over the phone, as previously agreed with Spain, that the issue should not damage UK-Spanish relations. A spokesperson added: "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. In the meantime, Mr Rajoy "committed to reducing measures at the border", and both leaders agreed that there should be a solution to the fishing dispute, the spokesperson added. Last weekend, Ambassador Giles Paxman sought an official explanation for Spanish threats to levy changes and to close airspace. Mr Paxman visited the Spanish secretary of state for foreign affairs Gonzalo de Benito. The Foreign Office said Mr Paxman reiterated that the UK would stand "shoulder to shoulder" with Gibraltar in the face of threats from Spain.