One said: "ZK027 is off radar now."
Another said: "I get alerts whenever a plane calls an emergency and a RAF Jet REG ZK027 has just stopped dead there, now there’s another Jet going towards it."
A third said the plane "has either landed or gone out of range wearing 7700".
An RAF jet has declared a general emergency off the coast of Wales (Image: AIRRADAR)
A 7700 squark is understood to be a common alert which could indicate an incident has occurred or there is a problem with the aircraft.
One plane spotter wrote on Twitter: "The Previously reported squawk 7700/7600/7500 aircraft "Unknown" ZK027 [British Aerospace Hawk T.2] has either landed or gone out of range wearing 7700."