Typhoon fighter jets should be fitted with collision warning devices to avoid a "catastrophic" mid-air crash, the UK's military aviation watchdog has warned the Ministry of Defence.
The Military Aviation Authority (MAA) said a crash between a Typhoon and a commercial jet was "improbable" but could cause "substantial loss of life".
Collision avoidance systems should be installed "with full haste", it said.
The Ministry of Defence said programmes to fit the systems had begun.
The MAA's advice follows a critical report issued last year into a mid-air collision between two RAF Tornados over the Moray Firth, off the coast of Caithness.
Three men were killed and another seriously injured in the incident in 2012.
Accident investigators concluded that the lack of a collision warning system onboard had contributed to the accident.
The MAA's annual air safety report published on Wednesday praised "good progress" by the MoD overall, but said failure to upgrade safety equipment on Typhoons was "unsustainable".
The director general of the MAA, Air Marshal Richard Garwood, said anti-collision software should also be fitted to the new F35 fighter.
AM Garwood said there were "well documented failings over the fitment of an airborne collision avoidance system (ACAS) to Tornado GR4".
He added: "In the worst case, which is judged improbable but catastrophic, a Typhoon colliding with commercial air traffic could result in severe consequences for the MoD because of the likely substantial third party loss of life."
Such a crash, he warned, could result in more than 50 deaths, and "would have adverse repercussions for the institution responsible, in this case the government of the day".
AM Garwood said: "Ultimately, the Typhoon will be in service for many years, and if the MoD elects not to fit an ACAS, the decision will need to be well documented and the risk held at the highest levels of defence."
The MoD said a collision avoidance system was being fitted to the Tornado GR4, the type of aircraft involved in the 2012 crash.
Work was also under way to have a system developed for Typhoons, the MoD said.
It added that "further technologies" and "planned upgrades" were proposed for the new F35 fighter.
An MoD spokesman said: "The department has welcomed the MAA's report, and is acting upon its comments and recommendations.
"Air safety is at the core of all our aviation activity and we take very seriously our obligations to our people and the wider community to ensure that our activities are both as safe as reasonably practicable and comply with relevant policy and legislation."
SNP Westminster leader and Defence spokesperson, Angus Robertson MP - whose constituency includes RAF Lossiemouth, home of Tornados and now Typhoons said: "This is a hugely damning report for the MoD.
"Twenty years after it was proven and recommended that these systems would save lives they remain to this day uninstalled.
"It is clear that the MoD with its cavalier approach to safety has learned no lessons as it has not even made the systems mandatory on new fast jets it acquires, while it drags its feet installing them on the ones they use already."