An 11-year-old boy boarded a flight to Italy on his own without a passport, tickets or boarding pass, Manchester Airport has confirmed.
The boy got through a security screen by mingling with a family going through Terminal 1 on Tuesday afternoon.
He was discovered while the Jet2 plane was in mid-air after passengers became suspicious.
A Manchester Airport spokesman said the "extremely serious matter" was being urgently investigated.
He said: "It is clear that documentation has not been checked correctly at security and the boarding gate.
"The boy went through full security screening, so the safety of passengers and the aircraft was never compromised."
It is believed the boy had earlier run away from home.
He was flown home on Tuesday evening.
BBC News - Boy, 11, boards plane to Italy at Manchester Airport without passport