Fighter jets have escorted the PIA aircraft from Manchester Airport to Stansted Airport, Sky News reported.
There are understood to be 297 passengers on board. The plane was heading west towards Manchester when it was suddenly re-routed near York and headed back out to the North Sea, before travelling south to Stansted.
An Essex Police spokeswoman said: "An incident has occurred on an aircraft. Police and partners are responding."
RAF Typhoon jets have been launched to investigate the incident involving a civilian aircraft within UK airspace, BBC quoted the Ministry of Defence as saying.
Media reports said the issue was a "security" incident. The reports said two RAF fighters were deployed alongside the Boeing 777 to intercept the plane travelling from Lahore to Manchester.