A man is in hospital and another is in police custody after a plane was forced to make an emergency landing.
Sources said the drama unfolded after a passenger was overheard claiming "something" was on board the flight in the luggage hold.
Once the aircraft touched down police boarded the aircraft and two British men, aged 49 and 57, were arrested.
A source said: "The plane was flying in when a passenger overheard a man saying there was something on the plane in the hold baggage. Obviously, they have to take it very seriously and go through their procedures and protocols and contact the police."
Police are treating the investigation as a criminal matter rather than a terrorism investigation.
The suspects, who along with other passengers departed for Heathrow from Colombo, were arrested on suspicion of endangering an aircraft.
The 49-year-old is being treated for a medical condition, not an injury, in hospital.
An Essex Police spokeswoman said: "The 49-year-old is currently receiving medical treatment in hospital and the other is in custody at Harlow police station.
"They will be interviewed by detectives. The incident is being treated as a criminal investigation. All passengers left the aircraft safely and have travelled onwards to Heathrow by coach."