The passengers had presented their boarding passes at the departure gate but were directed into a dead-end area, according to The Independent.
They had mistakenly gone down a set of steps instead of following the air bridge to the Airbus A321 aircraft and waited there for up to 40 minutes on Monday.
While they were still there, the aircraft took off, beginning its two-hour-and-45-minute flight to Alicante, without any crew member recognizing them missing, New York Post reported.
The seriousness of the situation only became clear when an airport employee rushed over and informed them that the flight had already departed.
Jet2 has apologized to the affected passengers and said it is urgently investigating the incident in coordination with Manchester Airport.
Similar incidents have occurred in the past.
On Jan 2, 2023, 23 passengers on a Ryanair flight from London’s Stansted Airport to Zagreb, Croatia, were trapped in a windowless corridor and missed their flight.