They arrested the German national on suspicion of entering a security restricted area without permission.
The flight was delayed for about an hour while passengers were rescreened.
A spokesman for Sussex Police said: "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 started.
"Airport staff found that she had no ID, no passport and no boarding pass.
"The plane was rescreened [searched] for security purposes and all the passengers were allowed to board and continue with their journey."
In a statement on its website, Easyjet said the flight was delayed for about an hour because of "additional security checks in Gatwick".
A statement from Gatwick claimed there was no risk to passenger security.
"As a precautionary measure, all passengers on flight EZY8703 disembarked the plane to be rechecked through security. We are investigating this matter with all relevant parties," it said.
EasyJet confirmed the flight was delayed by an hour and 15 minutes due to the additional security screening of all passengers.
"The passenger is now in police custody and the matter is now being investigated by the police and the airport," said a spokesman.
"EasyJet has apologised to all passengers for any inconvenience experienced."
JULIAN BRAY +44(0)1733 345581, Journalist, Broadcaster, Aviation Security & Airline Operations Expert, Travel / Maritime & Cruise Industry, EQUITY, NUJ, Broadcast COOBE ISDN ++44 (0)1733 345020 (DUAL CODEC) SKYPE: JULIAN.BRAY.UK e&oe Cell: 07944 217476 or iPhone 0743 530 3145 www.aviationcomment.com # # # VENDOR 10476453 http://feeds.feedburner.com/BraysDuckhouseBlog