Although British, Spanish and Gibraltarian authorities have said the navy´s arrival at the British overseas territory is long-scheduled, some in Spain see it as provocative.
At about 0715 GMT (3:15 a.m. EDT) the frigate HMS Westminster was seen approaching the port of Gibraltar flanked by two smaller ships.
Gibraltar´s creation of an artificial reef with concrete blocks, which Spanish fishermen say blocks their access to certain waters, has prompted Spain to toughen its border checks, leading to long queues for workers and tourists entering Gibraltar.
Spain claims the territory, population just 30,000, which it ceded to Britain by treaty 300 years ago.
While Spain has threatened to take its claim to Gibraltar to the United Nations, Britain has asked the European Commission urgently to send monitors to verify whether the border checks breach EU rules.