British Prime Minister David Cameron has warned his Spanish counterpart that the escalating tit-for-tat over border tensions in Gibraltar risked damaging relations between their countries.
Cameron and Mariano Rajoy discussed ways to calm the situation on Wednesday in a call Britain described as "constructive" after the row had escalated further at the weekend when Spanish Foreign Minister Jose Manuel Garcia-Margallo threatened to impose steep border tolls.
Constructive call with Spain's PM Rajoy. I made clear my concerns re Gibraltar and that our position on sovereignty won't change.
The British prime minister called Rajoy "to raise serious concerns about actions by the Spanish at the border with Gibraltar and suggestions... that they may introduce further measures," a Downing Street spokeswoman said.
"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," she said.
Cameron tweeted that he had "made clear his concerns on Gibraltar and that his position on sovereignty won't change''.
Gibraltar has accused Spain of deliberately creating border hold-ups in retaliation for the British overseas territory dumping concrete blocks in the sea to create an artificial reef.
Gibraltar says it wants to create an ecological reef, but Madrid claims it is a deliberate bid to impede Spanish fishing vessels in the dispute over territorial waters.
During the last weekend in July, Spanish border forces searched every vehicle entering the peninsula, creating delays of up to six hours.
Tensions rose further when Garcia-Margallo suggested Madrid could impose a $66 charge to cross the 1.2-kilometre frontier in either direction, which would affect the thousands of people who make the trip every day.
Spain ceded Gibraltar to Britain in perpetuity in 1713 but has long argued that it should be returned to Spanish sovereignty. London says it will not do so against the wishes of Gibraltarians - who are staunchly pro-British.