The Ministry of Defence called in Nato spy planes to help in the hunt, and at the height of the operation, five aircraft from four different nations with Royal Navy warships were involved in the search for the mystery vessel.

Those involved included France, Canada, and the US as the UK no longer has specialist planes for seeking out submarines. The hunt was said to have been launched after a periscope was spotted in waters where Royal Navy submarines normally surface as they head into or out of their base at Faslane late last month.

But it is unclear whether they succeeded in locating the submarine, or whether any foreign submarine had entered UK territorial waters, according to a report by Aviation Week.

The Ministry of Defence confirmed it had received assistance from Nato allies but would not say whether it had been searching for a submarine.

A MoD spokesman said: “We do not comment on the specifics of maritime operations, but can confirm that we requested assistance from allies’ forces to base maritime patrol aircraft at RAF Lossiemouth for a period.”