Britain has condemned what it called "provocative incursions" into waters around the British territory of Gibraltar by Spanish navy and customs ships, and summoned Spain's ambassador to express its concern.
In response Spain's Foreign Ministry said it had summoned the British ambassador in Madrid at almost exactly the same time to express concerns over recent events regarding its fishing boats in waters close to Gibraltar.
The spat is part of a long-running dispute over Gibraltar, a peninsula in southern Spain ceded to the British in 1713.
Britain's Minister for Europe David Lidington noted two "serious incursions" into Gibraltar's waters on Tuesday (local time).
"The UK has repeatedly made diplomatic protests to Spain over attempts by Spanish state authorities to exercise jurisdiction in British Gibraltar Territorial Waters," Lidington said in a statement.
"I condemn these provocative incursions and urge the Spanish government to ensure that they are not repeated," he said.
British Foreign Office Permanent Under Secretary Simon Fraser summoned Spanish Ambassador Federico Trillo, the Foreign Office said.
In May both countries agreed to work towards a solution to end months of skirmishes between the British territory's police and Spanish fishermen.
Earlier this year, Spain accused Gibraltar police of chasing its fishermen from Spanish waters and deployed Civil Guard officers to protect their boats.
Gibraltar and Britain said the police were enforcing fishing laws within their waters.
A spokesman for Spain's Foreign Ministry said on Thursday it would continue to protect the rights of its fishermen in the bay close to Gibraltar. It said it had summoned British Ambassador Giles Paxman.
Spain's Queen Sofia pulled out of a lunch to celebrate the diamond jubilee of Britain's Queen Elizabeth earlier this year as the dispute escalated.
Spain's centre-right government led by Mariano Rajoy has taken a strong line on Gibraltar, which houses traditional British red phone boxes, pubs and fish and chip shops.