An 11-year-old boy flew to Rome from Manchester Airport by himself without a passport or boarding pass.
Liam Corcoran passed through security without being checked, before making his way on to the Jet2.com flight yesterday.
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."
More top news
Freezing temperatures, a biting wind and risk of snow into next week
Austerity and Brexit are dividing Britons and causing a "schism" in society, the Archbishop of Canterbury has warned.
A taxi driver has been suspended after inspectors found he was using a Costa Coffee lid as a fuel cap.