An 11-year-old boy boarded a plane from Manchester to Rome on his own without a passport, tickets or boarding pass.
He mingled with families to get through checks in Terminal 1 on Tuesday. He was found mid-air on the Jet2 plane after passengers became suspicious.
The airline said no head count was carried out on this flight and they would now become mandatory.
A Manchester Airport spokesman said a number of airline staff had been suspended.
Transport Secretary Justine Greening said it was "incredibly concerning".
The youngster had been shopping with his mother at Wythenshawe Civic Centre when he went missing.
Staff at the centre were alerted and checked CCTV which showed the boy leaving the complex before he made his way to the airport.
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.
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."
Ms 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."
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.