A statement from the office of Prime Minister David Cameron said he told Spanish Prime Minister Mariano Rajoy there was a "real risk" of damaging relations between the countries if a controversy spurred by a fishing dispute was allowed to escalate.
"The PM made clear that our position on the sovereignty of Gibraltar and its surrounding water will not change," the statement added.
Rajoy agreed "that we needed to find a way to de-escalate the issue," the statement said.
The dispute has been brewing for weeks after Gibraltar fishermen dropped concrete blocks into the sea near the mouth of the Mediterranean Sea to create a reef. Spain claims the reef could block its fishing vessels.
Accusing Britain of encroaching on its territorial waters, Spain has created delays at border crossings that some travelers say have lasted six to seven hours, The Guardian reported.
Madrid also is considering imposing a $67 crossing fee to compensate Spanish fishermen who lost revenue because of the reef.
Officials of the British territory have accused Spain of using the border as a political weapon, The Telegraph reported.
"It's not about fish, it's about sovereignty," Gibraltar First Minister Fabian Picardo said. "The Spanish have always claimed rights to Gibraltar's waters that they don't have."
Spain insists the crossing fee is "allowable" under European Commission rules.
An EC spokesman said the fees must be "proportionate," adding the commission is reviewing their legality.