Flight training on RAF Typhoon jets has been "temporarily suspended" after safety inspectors found deposits of ash in one of the fleet's engines.
Safety inspectors took the "precautionary measure" to check all of the jets based at RAF Coningsby, in Lincolnshire, after finding small deposits on Wednesday, the Ministry of Defence said.
A spokesman added: "These are very high performance jets so they are just being extra cautious."
The ash was found on one of the jets which landed at the base yesterday, he said.
"They were flying as normal yesterday," he added. "Operational flying will continue."
The Typhoon is a multi-role combat aircraft, capable of being deployed for air policing and high intensity conflict.
The Typhoon Squadron at RAF Coningsby was launched in July 2005 after a multibillion-pound contract was agreed.
Britain, Germany, Italy and Spain formally agreed to start development of the aircraft in 1988, with contracts for a first batch of 148 aircraft - of which 55 are for the RAF - signed 10 years later.
The "highly capable and extremely agile aircraft" is powered by twin turbofans to Mach 2 at 65,000ft, according to the MoD.