The UK's aviation regulator should review the way it grants safety permits to ex-military aircraft following uncertainty over whether the Shoreham air disaster jet had a valid certificate, accident investigators have said.
Eleven people were killed when the Hawker Hunter crashed on to the A27 in West Sussex on August 22.
The Air Accidents Investigation Branch (AAIB) has published a special bulletin which revealed the lack of clarity over whether the jet should have been allowed to fly.
Aircraft that do not qualify for a standard certificate of airworthiness - generally former military or amateur-built planes - are able to operate with a permit to fly granted by the Civil Aviation Authority (CAA).
But the AAIB report revealed that initially the CAA "could not determine" if the Hawker Hunter met the requirements of its permit because of doubts over the validity of the maintenance of its Rolls-Royce engine.
A CAA spokesman said that since the bulletin was written it had informed the AAIB that maintenance of the Hawker Hunter jet was valid, with the next inspection not due until January.
The AAIB recommended that the CAA should "review its procedures" to ensure permits to fly are valid when they are issued.
Another safety issue raised by the Shoreham investigators came as a result of the pilot and his ejector seat being thrown clear from the cockpit.
The pyrotechnic cartridges fitted to the seat were still live and had been damaged in the crash, posing a "significant hazard" to emergency services.
The AAIB concluded that the CAA should require all operators of ex-military aircraft fitted with ejector seats to ensure contact details of someone who can make the devices safe after an accident are easily available.
It was also recommended that the regulator launches a review into the benefits and dangers of ejector seats. It is still unclear whether the Shoreham pilot - Andrew Hill, 51, from Hertfordshire - attempted to eject or if the force of the impact caused the movement of his seat.
The CAA issued a statement which read: "We have received the AAIB's latest special bulletin from its ongoing investigation into the Shoreham Air Show accident and will study it in detail.
"We will continue to fully co-operate with the AAIB investigation and provide expertise and advice."
"The AAIB special bulletin does not contain any information regarding the cause of the Shoreham air display accident."
The CAA is publishing its own review of air show safety which is due to be published early next year.
In the immediate aftermath of the crash the regulator banned vintage jets from performing "high-energy aerobatics" over land at public displays.
Mr Hill was questioned by Sussex Police last week under caution, but was not arrested.