EasyJet has been criticised for ordering an unaccompanied minor off an overbooked flight to Toulouse.

Casper Read, 15, was travelling alone from London Gatwick on Thursday 20 July on a trip to see his grandparents in France. The schoolboy had been accompanied to the airport by his mother, Stephanie Portal, who helped him check-in before he went through security.

Read boarded flight EZY8333 only to find another passenger had claimed his seat as the plane was overbooked. However, despite being an unaccompanied child, he was told he would have to leave. He was then left alone at the departure gate.

Portal received a text from her son saying he had been removed from his flight – fortunately she had not yet boarded her train.

“It’s crazy,” she told The Guardian. “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.”

The incident has prompted an investigation from easyJet. A spokesperson said: “We apologise for any inconvenience caused. 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.”

Read managed to board the last flight of the day to Toulouse after Portal convinced easyJet to secure a seat for him. The flight was scheduled to depart at 6.40pm but was delayed by nearly three hours. He arrived in Toulouse after midnight.

Every airline includes a clause in its ticket contract allowing it to deny boarding to any passenger it chooses. However, the airline must immediately compensate them, with the payment varying according to the length of the flight. Under 1,500km, for example London to Barcelona, the fixed rate is €250 (about £220); 1,500 to 3,500km, e.g. Edinburgh to Rome, €400 (£350); longer trips €600 (£525). But if the airline can get you to your destination within two, three or four hours respectively of the expected time, the compensation is halved.