The flight, AI 191, was on its way from Mumbai to New York's Newark airport when it turned back over Northern Ireland and was diverted to Stansted, north of London.
Two RAF Typhoon fighter jets were scrambled to intercept the civilian aircraft, the Ministry of Defence confirmed. "The aircraft was safely escorted to Stansted airport," a spokeswoman said.
In a tweet that has since been deleted, Air India said: "#Flight Alert! #AI 191 Mumbai-Newark of 27.06.19 has made a precautionary landing at London Stansted Airport due to bomb threat. Update in the matter follows."
Less than two hours after the diverted plane arrived at Stansted, the Reuters news agency quoted an Air India official as saying the threat had been a "hoax", and that the plane was already back up in the air.
But speaking to The Independent, a spokesman for Stansted denied this was the case. The Boeing 777 was "parked on an isolated stand here at Stansted and the police are continuing their investigations", he said.
Stansted is the UK's designated airport for aircraft facing active security threats to land. In a statement, the airport said it had briefly closed its runway to receive the plane but that it had since reopened and was now "fully operational".
"We are sorry for any disruption caused by the incident and would like to thank you for your patience," the airport said on Twitter, adding that "our first priority is always the safety of passengers and staff".
Dino Goel, a Twitter user who said he was a passenger on the flight and posted video purporting to show the plane being disembarked in London, said passengers were "currently being taken to the airport terminal after being deplaned, frisked and checked by sniffer dogs".
He said he had seen nothing untoward on the plane and that "I don't think the cabin crew even knew there was a threat". "Passengers are cool and calm so far," he said. "It's all good, everyone understands it's a security issue."
Social media users and local news outlets reported a loud boom being heard across a wide part of East England and the Midlands as the RAF Typhoon aircraft travelled at supersonic speeds.
Derybshire Police tweeted: "If you heard a loud bang over #Derbyshire today, this was as a result of a @RoyalAirForce jet passing over that caused a sonic boom."
The RAF spokeswoman said the Quick Reaction Alert (QRA) Typhoon aircraft were launched from RAF Coningsby in Lincolnshire.
"The Typhoon aircraft were authorised to transit at supersonic speed for operational reasons; any inconvenience caused to local residents is regretted," she said.
The Boeing 777 had taken off over three hours late from Mumbai. It is not clear whether that delay was connected with security concerns.
The aircraft was just south of Belfast at the time the decision was taken to divert. Evidently the situation was not regarded as extremely serious, because instead of landing at either Belfast International or Dublin, the plane flew back to Stansted – almost an hour's flying time.
Because of the diversion and subsequent delay, the crew are unlikely to be able to complete the flight to New York. Passengers will be transferred to other flights from Heathrow, or possibly carried on a replacement aircraft sent from India.
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