The HMS Talent has arrived in Gibraltar again, according to the UK's Defence Journal.
The UK nuclear submarine's arrival comes a month after a Royal Navy vessel fired a warning shot to signal to a Spanish coast guard vessel which was getting too close to the Talent. The sub was reportedly departing Gibraltar after having its Tomahawk missile stores replenished.
In a separate development in November, a Spanish customs boat was intercepted by a Gibraltar Defence Police vessel after it came too close to the USS Newport News, a Los Angeles class submarine.
"If classed as an incursion, the incident will almost certainly draw a diplomatic protest, as happens as a matter of routine with all incursions by Spanish state vessels," Defence Journal cited local media as saying.
Also last year, a Royal Navy ship had to chase a Spanish vessel as it tried to prevent a US warship from arriving in Gibraltar. The incident followed a Spanish patrol boat reportedly trying to "hassle" a US nuclear submarine attempting to dock at Gibraltar.
In late November, Fabian Picardo, the chief minister of Gibraltar, pointed out that Gibraltar was not interested to allow "even the remotest concession to Spanish sovereignty, jurisdiction or control".
The remarks came after British Prime Minister Theresa May stressed that London is committed to "always stand by Gibraltar", and that Britain "will always negotiate on behalf of the whole UK family, including Gibraltar".
Gibraltar is set to leave the European Union together with the rest of the UK in March 2019. The Brexit agreement between London and Brussels includes a special protocol on Gibraltar.