An intruder managed to board an easyJet aircraft at Gatwick with no passport or boarding pass before the departure gate had opened.
Police were called when a 51-year-old German woman boarded the plane to Tenerife just after 7am on Friday without a ticket or identification documents.
She was arrested on suspicion of entering a security restricted area without permission.
The flight to Tenerife was delayed for more than an hour while passengers were rescreened.
A Sussex Police spokeswoman told the BBC: "Police were called to the North Terminal departure gate 111 in Gatwick airport ... to a report a woman had boarded an easyJet flight to Tenerife before boarding had actually started.
"Airport staff found that she had no ID, no passport and no boarding pass.
"The plane was rescreened for security purposes and all the passengers were allowed to board and continue with their journey."
EasyJet said the flight was delayed for about an hour because of "additional security checks in Gatwick".
An airport spokeswoman said "there was no risk to passenger security" and it was investigating the matter "with all relevant parties".
"As a precautionary measure, all passengers on flight EZY8703 disembarked the plane to be rechecked through security," she added.
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