A man was escorted off an Easyjet plane at Naples airport in Italy after ammunition was found in his hand luggage by cabin crew.
Easyjet said the man should have been detected by airport security
The flight to Stansted carrying 120 passengers was delayed after the elderly traveller, thought to be Italian, was stopped before take off.
It is thought he mistakenly packed the rounds in his bag.
Easyjet said the man was not a "threat" but it was unacceptable that he was able to get past airport security.
Flight EZY3506 took off two hours late at 2230 local time after a full security check was carried out.
The man and his wife were questioned by airport officials.
Ammunition can be transported on flights in some instances but must be checked into the hold and placed in a metal or wooden box.
Easyjet said the man was approached by staff as people took their seats on the plane but none of the other passengers became scared.
"The crew noticed one particular passenger who looked as though he had something suspicious in his bag," an Easyjet spokeswoman said.
"It turns out he had some ammunition in his bag. It was a couple of rounds of ammunition. He didn't have any kind of weapon."
She said he appeared confused about what had happened.
"We believe that he simply didn't realise ammunition cannot be packed in hand luggage."