Britain is making a formal protest in Madrid after a Spanish warship entered the waters around Gibraltar and disrupted a British forces parachute training exercise.
The UK's ambassador in the Spanish capital will raise the incursion at "a high level" in government as tensions increased yet further over the disputed territory.
Military chiefs were forced to suspend the session while the Gibraltar Squadron's patrol boat, HMS Sabre, shadowed the Spanish navy ship.
Warnings were issued to the vessel, which is said to have used a fake name and call sign, but it only left after radioing back that it was in Spanish waters, the Gibraltar Chronicle reported.
A Foreign Office spokesman said: " There was an incursion into British waters near Gibraltar by the Spanish Navy on 18 February, during a Royal Navy exercise. The exercise went ahead irrespective of this incursion.
" We are raising this at a high level with the Spanish government and will be issuing a formal protest. Neither this incident nor other incursions change the fact of UK sovereignty over British Gibraltar Territorial Waters."
Relations have been strained since the construction of an artificial reef by the Gibraltar government last year which the Spanish said interfered with their fishermen.
Madrid responded by imposing tighter border controls, leading to long delays at the frontier.
Repeated incursions into Gibraltar's waters by Spanish state vessels followed, which led to the Spanish ambassador being publicly summoned to the Foreign Office on a number of occasions.
In November, a British diplomatic bag was opened and searched by Spanish Guardia Civil officers on the border with Gibraltar - an incident the Foreign Office described as a ''serious infringement'' of international diplomatic protocols.
A Ministry of Defence spokesman said: "The Royal Navy patrols and continues to challenge all unlawful incursions into British Gibraltar territorial waters by Spanish state vessels by issuing appropriate warnings.
"This forms an important part of Her Majesty's Government's commitment to uphold our sovereignty against unlawful incursions with a range of proportionate naval, police and diplomatic responses.
"There should be no doubt that the MoD remains absolutely committed to ensuring the defence and security of Gibraltar, its residents and British Gibraltar territorial waters."
It is understood that reports locally suggesting that special forces personnel were involved in the training session were wide of the mark.