IndiGo flight 6E 3612 was grounded at Mumbai Airport after a bomb threat (Representational)

An IndiGo flight from Mumbai to Lucknow via Delhi was grounded this morning after a bomb threat call, airport officials said.

The flight, with 169 passengers on board, was scheduled to depart at 6:05 am, they said.

"A passenger travelling with another private carrier told our staff at the Mumbai airport that there could be a bomb on the Mumbai-Delhi flight 6E 3612. IndiGo reported the matter to the authorities concerned and followed the normal security protocol," an IndiGo spokesperson said.

"The passenger was found to be mentally unsound. Operations have resumed. However, the flight was delayed by one hour," the spokesperson said.

An airport source told news agency PTI that "A woman passenger travelling to Delhi on Go Air flight G8 329 approached an IndiGo check-in counter at T1 and informed that there was a bomb in flight 6E 3612 (operating on Mumbai-Delhi/Lucknow route)."

She also showed photographs of a few people and claimed that they were a "threat" to the nation, the source told news agency PTI.

CISF security personnel took her to the airport police station for questioning.

The aircraft was taken to an isolated bay for a secondary ladder point check after a Bomb Threat Assessment Committee (BTAC) found the threat as "specific", the sources added.

Later, security agencies declared the plane "safe", they said.

