LONDON, May 24: British authorities scrambled fighter jets on Friday to intercept a PIA airliner carrying 308 people from Pakistan to Manchester and diverted it to an isolated runway at an airport on the outskirts of London. Two British passengers of Pakistani origin who had allegedly threatened to destroy the plane were arrested.

A British security official said the situation involving the Pakistan International Airlines flight did not appear terrorism-related, though police were still investigating. However, the incident further rattled the UK just days after a soldier was killed on a London street in a suspected terrorist attack.

A Pakistani official briefed by the British police and PIA security on the investigation said on condition of anonymity the two suspects, speaking Urdu, had allegedly threatened to “destroy the plane” after an argument with the crew.

Flight PK709 was diverted by the fighter jets to Stansted Airport.

By strange coincidence, the PIA plane diverted to Stansted was the same plane on the same route — Lahore to Manchester — that had been diverted to Stansted in September 2011 due to a bomb scare.

A spokesman for the British Ministry of Defence said that Typhoon jets had been launched from a Royal Air Force base after the incident was signalled by the plane’s crew, shortly before the plane was due to land in Manchester at 1230 GMT.

“Typhoon aircraft from RAF Coningsby have been launched to investigate an incident involving an aircraft in UK airspace,” the ministry spokesman said.

After the plane landed, he said the incident was now a police matter and “our involvement is over”.

Typhoon planes can be scrambled if the pilot or crew of a passenger aircraft sends out a passenger signal, he added. “The purpose of going up is to investigate what the situation is,” he said.

“Often when a Quick Reaction Alert aircraft is launched the details are not known, but it is known that a signal has been sent.”

The incident came just hours after a British Airways plane was forced to make an emergency landing at Heathrow Airport with smoke billowing from one of its engines.

Heathrow was temporarily forced to close both its runways while emergency crews put out a fire on the Oslo-bound Airbus A319, causing heavy disruption at one of the world’s busiest airports. A British Airways spokesman said the incident was “a purely technical issue”.

Police said the two men were detained on suspicion of ‘endangering’ the aircraft.

“Two men have been arrested on suspicion of endangerment of an aircraft,” police in Essex said in a statement. “They are aged 30 and 41 and are being taken to a police station for interview by detectives.”

A PIA source said the incident had stemmed from a family row on board. “There was a family of eight to 10 people on the plane and they were quarrelling among each other,” the source said.

“When PIA staff approached them and asked them to calm down, they told them to go away otherwise they would blow up the plane.

“PIA staff became scared and they raised the alarm to avoid any untoward situation.”

Passenger Nauman Rizvi told a Pakistani TV channel that the two men who had tried to move towards the cockpit were handcuffed and arrested once the plane landed. Mr Rizvi said that after the men were taken away, the flight crew told passengers there had been a terrorist threat and that the pilot had raised an alarm.

The plane will be examined by forensic specialists but no suspicious items have been recovered so far, police added.

“This incident is being treated as a criminal offence,” the police statement said, in another indication it was not being seen as a terrorist case.—Agencies

Bhagwandas adds from Karachi: PIA spokesperson Mashhood Tajwar said the crew informed the pilot about the trouble, who immediately conveyed a message to the Air Traffic Controller. The ATC rushed the RAF to escort the plane to Stansted Airport.

He said the passengers were at the Stansted Airport and soon after security clearance by the UK authorities the aircraft would fly for Manchester.

Mr Tajwar said that all the people on board, 207 passengers and 11 crew, were safe, adding that passengers and their baggage were being screened.