RAF pilots will have to fatten up to fly Britain’s new fighter aircraft after tests showed ejector seats could cause injury at low speeds.
Any flying officer weighing less than 136lbs is banned from getting into the cockpit of the Lockheed Martin F-35 Lightning jets.
Defence Minister Philip Dunne confirmed the temporary weight restriction had been put in place.
It follows a similar move in the US, after tests revealed an increased risk of neck injury if the Martin-Baker ejector seat carried a lightweight pilot and was deployed at low flying speeds.
The £100 million stealth fighters will become airborne from the decks of Britain’s new aircraft carriers by 2020.
Mr Dunne told parliament: “A temporary weight restriction has been imposed for UK pilots, which does not constrain flight operations at this time.
“A range of design solutions are being developed and these are expected to be in place before the aircraft enters service in the UK.”
An MoD spokesman said: “There are no UK F-35 pilots affected by this issue and we do not anticipate any impact on national flying operations while this seat development issue is resolved.
“The safety of our personnel remains of paramount importance.”
The jet’s distinctive shape and radar absorbent coatings, give it the lowest possible radar signature.
Experts say it renders it almost invisible to high frequency radars used in modern air-defence systems.
Like the Harrier Jump Jet it replaces, the Joint Strike Fighter stealth jet will be able to land and take off vertically and will fire air-to-air missiles and laser-guided bombs.
The US, Italy, Canada, Turkey and Australia will also have the planes.