A British Airways flight was intercepted by fighter jets over Hungary after the passenger plane lost contact with air traffic controllers.
Hungarian authorities reportedly issued their highest alert after the Boeing 777 passed over its border unannounced on Saturday afternoon.
Two Hungarian Air Force Gripens were scrambled to identify the aircraft which was en route from Dubai to Heathrow.
According to air safety protocols, pilots must make contact with air traffic controllers on the ground when passing from one country's airspace to another.
The Gripens reportedly took off at 12.55pm before the BA flight made contact with ground control.
A BA spokesman said: "Communication was quickly restored with air traffic control and the flight landed normally at Heathrow."
There have been several similar incidents recently where civilian commercial flights have inadvertently triggered high-level security alerts.