The Ministry of Defence said two Typhoon fighter jets from RAF Coningsby escorted the Pakistan International Airlines flight to Stansted Airport.
A Manchester Airport spokesman said the Pakistan International Airlines (PIA) flight had been due to land at 1:30pm, but was diverted to London Stansted "where the authorities are investigating".
The spokesman said: "The plane - flight number PK709 - was travelling from Lahore and had 297 passengers on board.
"For further information for friends or family who were due to meet the flight at Manchester please call 0800 140 4007".
Stansted Airport said its flights are "operating as normal" after a passenger plane was diverted there following an incident on board:
Flights are operating as normal @stn_airport following an earlier incident with an aircraft.
– essex police statement
Essex Police has boarded a passenger plane diverted to Stansted Airport and two men have been arrested on suspicion of endangerment of an aircraft.
They have been removed from the plane.
Police and partners are continuing to respond
The Guardian reports that the pilot of the plane that has been diverted to Stansted Airport asked for this to happen because he was concerned about a disruptive passenger who had started shouting.
The pilot then became satisfied that the incident had been resolved, the newspaper reports.
Eurocontrol, Europe's air navigation safety watchdog, said "significant" delays are expected at Stansted Airport following an "incident with a 777 declaring an emergency":
Stansted Airport has said that the diverted plane is in an isolated stand away from passenger areas.
It added that the airport is operating as normal.
A British security source has told Reuters news agency that early indications do not suggest that the plane is the subject of a terror attack.
It is understood the incident on board the Pakistan International Airlines flight PK709 happened around 10 minutes before the plane was due to land in Manchester at 2pm.