He said: “The pilot was recovered safely and is still undergoing medical checks. We are hopeful that he will be absolutely fine. It would be premature of me to comment on the reasons for the accident.

“The recovery of the flight data recorder and the wreckage are really vital for an accurate investigation to determine the causes of the crash. Clearly the swift recovery of the aircraft is what we would like to do and we are working closely with allies on the mechanics of that. We haven’t got the plane up yet.

“We are aware of Russian undersea capabilities, and you are quite right to identify them as being state of the art. 

“The kinds of precautions and operations that we are undertaking at the moment are designed at least in part to ensure that the technology of the F-35 remains as confidential as you would like it to be. 

“Those security aspects are very much at the top of our mind.

“My understanding is that the experts know where the aircraft is.”

There will ‘never be a good time’ for Ukraine invasion

Sir Stephen was also questioned about fears that Russia could be planning an imminent invasion of Ukraine.

“What I would say to anybody in Russia considering this is there is never going to be a good time to make an incursion into Ukraine,” he said. 

“They should not make the mistake of interpreting a withdrawal from Afghanistan as a lack of willingness on the part of Western allies to stand up for sovereign democracies, wherever they are in the world but certainly in mainland Europe.

“I am aware that Russian actions sometimes seem to be driven by an opportunistic type of motivation, and I would counsel them very, very clearly not to fall into that trap now.

“I can absolutely assure the committee and my Russian counterpart that there will be considerably more activity than a mere condemnation.”