According to The Sun, the VC-10 tanker was scrambled from RAF Brize Norton, Oxon, to take air samples near the rogue state with an underwing pod.
Any fallout gathered will tell scientists what sort of nuclear material was used in Monday's Hiroshima-sized underground blast.
A military source said yesterday: "Once that is known, an estimate of the bomb's strength or 'yield' can be made."
The flight crew is under strict orders not to approach any closer to the North Korean coast than 50 miles for fear of being shot down. (ANI)