Britain has summoned the Spanish ambassador after Spain sent a ship into Gibraltar's waters in a move that has reignited tensions over the territory.
In what is being seen as a clearly inflammatory move Spain sent a state research vessel accompanied by a police boat to the waters off the British territory on Tuesday.
Europe Minister David Lidington said the activities of the vessels were both "unlawful" and some of the manoeuvres made by the Guardia Civil boat were dangerous.
He vowed to take "whatever action necessary" to safeguard the interests of Gibraltar.
A Spanish ship refused to leave Gibraltar's waters in NovemberThe Spanish Ambassador Federico Trillo was summoned to the Foreign Office on Wednesday to explain the "serious incursion".
It is the fourth time he has been publicly summoned by Britain over Gibraltar since December 2011, when the current Spanish Government took office.
Sky's Diplomatic Editor Tim Marshall said the move by the Spanish suggested they had "fully intended to provoke the British."
He added: "This is not to say who is right and who is wrong in law. The British are extraordinarily confident that in law these are British territorial waters and the Spanish have got no rights there in that they are supported by most of the European Union."
Mr Lidington said: "Not only were the actions of the survey vessel unlawful, but it was accompanied by a Spanish Guardia Civil vessel whose dangerous manoeuvring presented a significant safety concern on the waters.
Travellers faced lengthy queues at the Gibraltar border in the summer"I strongly condemn this provocative incursion and urge the Spanish government to ensure that it is not repeated. Her Majesty's Government will continue to take whatever action we consider necessary to uphold British sovereignty and the interests of Gibraltar, its people, its security and economy."
The Prime Minister's spokesman said: "(We) raise very clearly our concerns with the Spanish government. We will continue to do that if we need to. We would strongly urge the Spanish authorities to desist."
It is the latest incident in a long-running row between Spain and the British territory.
In November Mr Trillo was summoned after a Spanish ship refused to leave Gibraltar's waters for 20 hours.
In the summer Spain tightened up its checks on travellers crossing the border to Gibraltar leading to delays of several hours following a diplomatic row over an artificial reef built by Gibraltar provoking anger from Spanish fisherman.
David Cameron asked the European Commission to investigate claiming the checks were "politically motivated" and potentially in breach of EU law on free movement of people.
However, the commission said the checks were not unlawful and ordered the UK and Spain to work together.
In August Spanish mayor Francisco Perez Trigueros provoked anger in Gibraltar after posting a mock-up picture of Spain invading The Rock on Facebook.
Spain lays claim to Gibraltar, which has a population of just 30,000 and was ceded to Britain 300 years ago.