Britain is to summon Spain's ambassador after what it described as a "provocative incursion" into Gibraltar's territorial waters by a Spanish vessel, in the latest round in the war of words over the British outpost.
The Foreign Office said on Tuesday that a Spanish government research ship had entered Gibraltarian waters on Monday and refused orders from the Royal Navy to leave.
The Europe minister, David Lidington, said: "I strongly condemn this provocative incursion and urge the Spanish government to ensure that it is not repeated. We stand ready to do whatever is required to protect Gibraltar's sovereignty, economy and security."
The standoff began when the 164ft RV Ramon Margalef carried out what its captain described as "oceanographic works in the interest of the European community" at the behest of the Spanish government.
A tiny British patrol boat closed in on the Spanish vessel, ordering it to leave Gibraltar waters, but to no effect. The Spanish ship finally left the area after almost 24 hours.
On Tuesday Gibraltar's government issued a statement denouncing what it described as an "intolerable intrusion" and a clear violation of the UN convention on the law of the seas.
"The government takes a very serious view of this development, which represents yet another escalation of Spain's campaign against Gibraltar on land and at sea," said a government spokesperson.
Earlier in the summer, Spain and the UK engaged in a series of public rows over Gibraltar, with the former accused of deliberately tightening border control checks in order to impede those crossing between the two sides.
David Cameron complained to the Spanish prime minister, Mariano Rajoy, and called on Brussels to carry out an independent inspection of the border crossings. Last week, the European commission reported it had found no evidence of Spain infringing EU border regulations.
Many believe it suits the government in Madrid to make Gibraltar a focus point while Spain endures a crippling economic crisis, numerous cases of corruption at the heart of the government and an ongoing crackdown on civil liberties.
This latest spat will be the third time London has called in the Spanish ambassador since Rajoy's rightwing government was formed in 2011.
During Spain's decade-long economic boom, which came to an end in 2008, Gibraltar rarely featured so heavily in the headlines, but as Spain's economic situation continues, so the regularity with which it has become an issue has increased.
On Tuesday Cameron's official spokesman said: "Our view about the importance of the territorial integrity of our sovereign waters is unchanged. It is very important and we have communicated that to the Spanish government. Our relations with the Spanish government of course are important to us and we work with them in a number of areas, but where we have differences we make these very clear and we will continue to do that."
In a previous incident this month, the British government issued a formal complaint to Madrid following a standoff at sea between a Royal Navy ship and guardia civil patrol boat.
The overseas territory of Gibraltar, with a population of around 30,000, was ceded to Britain by Spain under the treaty of Utrecht in 1713.
In referendums held in 1967 and 2002, Gibraltarians widely rejected proposals for it to be governed by Madrid, but Spain continues to press its claims to the territory.
