An 11-year-old boy boarded a plane from Manchester to Rome on his own without a passport, tickets or boarding pass.
The boy got through a security screen by mingling with families 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 a number of airline staff had been suspended. The transport secretary said it was "incredibly concerning".
The airport spokesman said the boy passed through security checks but was not asked to show any documentation.
The boy went straight to the nearest aircraft that was boarding - the flight to Rome - where he was also missed during a head-count on board the plane.
The spokesman 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."
Transport Secretary Justine Greening said: "I treat security breaches very, very seriously indeed, so we are now reviewing urgently with Manchester Airport, and indeed the airline, exactly what happened."
A spokesperson for Jet2.com said: "On Tuesday an 11-year-old boy cleared security at Manchester Airport, without the necessary paperwork but had been through a full security search.
"The boy then boarded our flight bound for Rome.
"We have launched a full investigation into what is a serious incident, and the staff involved have been suspended pending the outcome.
"The boy has been returned safely to his family."
It is believed the boy had earlier run away from home.
He stayed on board the plane when it landed at Rome Fiumicino Airport and was flown back to Manchester when the plane returned on Tuesday evening.