Arabic graffiti has been found spray-painted on four easyJet aircraft and one from Vueling at two French airports.
Three defaced aircraft were found in Lyon and two at Charles de Gaulle, the AFP news agency reported.
'Allahu Akbar' - arabic for 'God is great' - was scrawled on a fuel-tank hatch of one easyJet aircraft in Paris.
The discovery came as it was reported that the security passes of 86,000 workers at Charles de Gaulle airport will be reviewed after it was found that 57 employees with access to aircraft were on a terror watch list.
Security badges were taken away from dozens of workers at the airport after the Charlie Hebdo terror attacks in Paris in January - but others continued working, the Sunday Times reported.
Police official Philippe Riffault told the newspaper that the review of airport passes will begin with 5,000 security personnel.
EasyJet 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.
“Our security team is experienced at assessing any potential threats and following this assessment this is not considered to be a security issue by both us and the authorities who share our assessment that it poses no risk whatsoever.
“The crew would not start boarding the aircraft unless they were completely satisfied that it was safe to do so and the crew can take whatever time they deem necessary to carry out security assessments. We do not compromise on safety.
“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.”
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