Officials from Gibraltar have confirmed that a Spanish gunship has entered British waters off the Rock in a second illegal move to antagonise Britain.
A Spanish Navy ship called Infanta Cristina embarked upon a similar move last week, where the Royal Navy forced it to leave.
These events emerge amid increasing Brexit tensions over the Rock's future.
Last Tuesday, the Foreign Office said the Royal Navy 'successfully challenges all unlawful maritime incursions into British Gibraltar Territorial Waters.'
However, Spain's foreign ministry refuted the claim, saying the ship was sailing through Spanish territorial waters.
Both Prime Minister Theresa May and Foreign Secretary Boris Johnson confirmed to the people of Gibraltar that the British Overseas Territory will remain British once Brexit has been fully implemented.
Mrs May would 'go to war'
Gibraltar's Chief Minister, Fabian Picardo, has said he remains committed to the Rock remaining as a British Overseas Territory.
The Infanta Cristina entered British waters last Tuesday lunchtime as part of a venture the Gibrlatar Government said was an 'illegal incursion.'
But it soon left the waters after it was challenged by the Royal Navy's HMS Scimitar.
This incident came a day after the Spanish Government suggested the British should 'calm down' over Gibraltar's future.
Spanish Foreign Minister Alfonso Dastis made the remarks in response to former Conservative leader Michael Howard's comments over the Rock.
The former Tory home secretary said on The Sunday Politics Mrs May is willing to 'go to war' over the British Overseas Territory's future.
'Like a cuckolded husband'
An EU document suggested Spain would be provided with a veto on post-Brexit agreements regarding Gibraltar, meaning they could use upcoming negotiations as an opportunity to deny Britain access to the Single Market if it does not surrender its sovereignty over the Rock.
Mr. Picardo, said the President of the European Council, Donald Tusk, was behaving like 'a cuckolded husband who is taking it out on the children' for allowing Spain to use Brexit as an opportunity to settle its historical claims to the Rock.
US President Donald Trump also provided his support for the Prime Minister's position on Gibraltar.
However, Lord Howard compared Mrs May to his former boss, Margaret Thatcher, who went to war to defend the Falkland Islands from an Argentinian invasion in 1982.
Following the former Tory leader's comments, former UKIP leader Nigel Farage said the EU had made an 'absolutely catastrophic error' and Britain would not be intimidated.
'A UN breach'
A spokesman for HM Government of Gibraltar said Infanta Cristina's incursion was a 'breach of the UN Convention on the Law of the Sea.'
But it remains unclear which Spanish ship carried out today's incursion.
This event represents the emerging difficulties the Prime Minister will have in trying to secure Gibraltar's future as a British Overseas Territory once Britain prepares to negotiate its way out of the European Union.
It also adds to the sense of anxiety people travelling to Spain from Gibraltar, and vice versa, for work face over the complexities of negotiating a successful outcome for both sides.
A lack of certainty over how Brexit would affect Gibraltar's status resulted in the Remain side winning a 96% share of the vote in last year's EU Referendum.
The Rock has had three referendums in 1967, 2002 and 2013 to decide whether or not it wants to remain British, and in each one, the people of Gibraltar voted overwhelmingly in favour of remaining as a British Overseas Territory.
Spain lost Gibraltar to the British in 1704 during the War of Spanish Succession. The Rock's status as a British colony was confirmed by the Treaty of Utrecht (1713) which ended the conflict.
However, the Spanish attempted to retake Gibraltar during the American Revolution (1775-83) where it fought a three year battle over the Rock (1779-1782) and lost.
The British also managed to prevent the Spanish from reclaiming the Rock during the Napoleonic Wars when Spain allied with the French in 1804.
More follows on this story.
