A Spanish warship caused a training exercise by British forces to be suspended after entering the waters around Gibraltar.
The Gibraltar Squadron's patrol boat HMS Sabre shadowed the ship, which is said to have used a fake name and call sign, the Gibraltar Chronicle reports.
Royal Navy special forces personnel are believed to have been carrying out a parachute jump at the time, the newspaper said.
Warnings were made to the vessel, which only left the area after radioing back that it was in Spanish waters.
In November, the Foreign Office condemned an incident in which a British diplomatic bag was opened and searched by Spanish Guardia Civil officers on Gibraltar's border with Spain.