Image caption The UK called on help from the US, France and Canada to search for the submarine

Britain called on the help of aircraft from Nato allies after a reported sighting of a submarine periscope off the west of Scotland last month.

The search continued until last week, with planes from the US, France and Canada flying out of RAF Lossiemouth.

Britain no longer has its own fixed-wing aircraft specifically designed to search for submarines.

The Ministry of Defence would not confirm it was looking for a foreign submarine.

But there has been an increase of Russian military activity in recent months.

In a statement the MoD said it "requested assistance from allies' forces for basing maritime patrol aircraft at RAF Lossiemouth for a limited period".

Maritime nation

Scottish National Party Defence spokesman Angus Robertson, whose north-east Scotland constituency contains RAF Lossiemouth, said: "This is hugely embarrassing for the UK which is totally exposed without such critical maritime patrol assets.

"It is not the first time they have had to depend on the goodwill of allies to fill this massive capability gap."

The SNP MP for Moray added: "It is absurd that Scotland as a maritime nation doesn't have a single maritime patrol aircraft.

"All of our surrounding neighbours have them. It is utter madness that the MoD are going to renew Trident at a cost of £100bn but not on essential conventional kit like these aircraft."

At the height of the search, two US Navy P-3 Orions, a Royal Canadian Airforce Aurora and a French Dassault Atlantique were involved.

An RAF Sentinel spy-plane and a Royal Navy warship also took part.

Patrol boat

The incident comes more than a month after Sweden halted its search for what was believed to be a foreign submarine operating in its territorial waters.

On 28 November, a Royal Navy patrol boat shadowed four Russian navy warships on transit through the English Channel.

At the time the Navy said that it did not send a warship as the smaller patrol vessel HMS Tyne was adequate for the job.

The BBC now understands the fleet escort warship assigned to protect Britain's territorial waters was taking part in the search for the suspect vessel off the coast of Scotland at that time.