A French patrol plane has been scouring the seas off the Scottish coast for a Russian submarine at the request of the government as Britain no longer has sub-hunting aircraft.
The Russian vessel has evaded detection for at least 10 days after it was detected off the coast, according to The Telegraph.
The submarine is feared to be spying on Trident, the UK's nuclear deterrent.
The search intensified over the weekend as a second a French Atlantique II maritime patrol plane and a Canadian aircraft joined the hunt.
The Royal Navy frigate HMS Sutherland, and a hunter killer submarine, are also patrolling the area.
But Britain's allies were called on to search for the sub from the air as the UK scrapped its own Nimrod sub-hunting aircraft in 2010 following a defence spending review.
That decision was controversial at the time with defence chiefs warning it could leave the country dangerously exposed.
The Ministry of Defence has not commented on the hunt for the Russian submarine.
An MoD spokesman said: "We can confirm that allied maritime patrol aircraft based at RAF Lossiemouth for a limited period are conducting activity with the Royal Navy.
"We do not discuss the detail of maritime operations."
The MoD previously requested US help to search for a suspected Russian submarine off the Scottish coast in January.
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