Arabic graffiti was found daubed on panels covering fuel tanks of easyJet planes at French airports.
An investigation is being carried out into who made the inscriptions, which were written on four planes in France. The airports cannot be identified for security reasons.
Lisa King, easyJet cabin safety manager, alerted company employees to the graffiti with an email informing them of the discovery of "four aircraft in France with written inscriptions on the inside of the fuel panel, and toilet door in Arabic script."
A spokeswoman for easyJet said the matter was not considered to be a threat and no passengers had to be removed from the aircraft.
She said: "EasyJet assessed this issue, each time working in full consultation with the authorities, and is entirely satisfied it is nothing more than graffiti.
"EasyJet takes very seriously any security related issue and would not operate a flight unless we are entirely satisfied it is completely safe to do so.
"EasyJet operates its fleet of aircraft in full compliance with all regulations. The safety and security of its passengers and crews is always easyJet's highest priority."
The graffiti was found less than two weeks after 130 people were killed in terror attacks in the country's capital.
Philip Baum, editor of Aviation Security International, told The Sun: "Graffiti in itself won't hurt anybody. But the ability of anyone to place a prohibited item near fuel tanks is a concern, of course."
"We know there are people working in restricted areas of airports with extremist sympathies."