An 11-year-old boy took a flight to Rome on his own from Manchester Airport without a passport or boarding pass.
Liam Corcoran passed through security without being checked, before making his way on to the Jet2.com flight.
A Manchester Airport spokesman said: “This extremely serious matter is now being urgently investigated by officials from the airport and airline.
“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.
“We are now making preparations to ensure that his return to the UK is handled sensitively to avoid any distress.”
Liam had run away from his mother during a shopping trip at Wythenshawe Civic Centre on Tuesday and made his own way to the airport, according to the Manchester Evening News.
The newspaper reported that he followed a family at Terminal 1. He was scanned by security staff, but his lack of documents was not noticed.
The youngster passed through a gate where he was not required to show a passport or boarding pass, and made it on to Jet2.com flight LS791 to the Italian capital without being asked for a boarding pass stub by cabin crew.
The Manchester Airport spokesman said that once passengers became aware that the boy was on his own, airline staff were alerted and the plane’s captain informed the authorities.
He said the boy remained on board after landing at Rome Fiumicino Airport, and returned to Manchester on the same plane, where he was met by his mother at around 9pm on Tuesday.
A number of staff from Manchester Airport and Jet2.com, which is based at Leeds Bradford Airport, have been suspended while the investigation takes place.
Speaking on BBC News 24 today, Transport Secretary Justine Greening said: “This is an unusual and serious breach and we are keen to find out what has gone on.”