The airline EasyJet has started an investigation into how an unaccompanied child was removed from a flight and left alone at an airport.

The 15-year-old was flying aboard flight EZY8333 from London Gatwick to Toulouse on Thursday 20 July, but when the plane was overbooked he was taken off and left in the departure lounge.

Casper Read, from Worthing, West Sussex, had planned on staying with relatives in France in what was only his second unaccompanied flight.

He had been taken to the airport and checked in by his mother before they parted at the security gates.

Read had been assigned seat 9A. But there was someone already in his seat and then he was told to leave the plane.

His mother Stephanie Portal arrived back at the airport and eventually convinced the airline to put her son aboard their final flight to Toulouse for the day. She told the Guardian that all four flights that day had been overbooked.

Portal said: "It's crazy. They left him alone in departures. Luckily, I had still not got on board my train to London and could come back and find him. If I had not been there I don't know what would have happened – he'd have had no money for the train back or anything."

Responding to the incident, a spokesperson apologised, saying: "EasyJet is sorry that Casper Read's flight from London Gatwick to Toulouse was overbooked. We are investigating why he was able to board the aircraft as he should have been informed at the gate.

"EasyJet has a procedure to protect unaccompanied minors but unfortunately this was not followed on this occasion and so this will also be investigated."

Since a man was forced off a united Airlines flight in March, airliners have been put under heavy scrutiny on overbookings and how they handle individual passengers.