New 5G phone technology could interfere with aircraft signals and affect altitude instruments that are critical for landing.
Phones that use the technology should be switched off during flights, the French Civil Aviation Authority said on Tuesday.
"The utilisation of 5G devices onboard aircraft could lead to risks of interference that could potentially result in errors in altitude readings," a spokesman said.
The potential danger is because of "signal interference from a close frequency source of a strength that is similar or even superior to that of altimeters."
This interference can cause errors "in instruments that are extremely critical during landing," said the agency, known by its French acronym DGAC.
It sent a bulletin on the issue to airlines last week, recommending that 5G phones should either be turned off completely or switched to "airplane mode" during flights.
Most countries have long required that mobile phones be turned off or placed in flight mode because of concerns that previous generations of mobile telecommunications networks can interfere with a plane's navigation and communication equipment.
The DGAC also recommended in cases of disruption to aircraft equipment that the flight crew immediately notify air traffic controllers who can then alert airport authorities.
DGAC said that it had laid out conditions for the positioning of 5G base stations in order to limit the risks of interference during landing at French airports.
The strength of signals from 5G base stations placed near France's main airports has been restricted, said the DGAC, which has been conducting additional testing since November when French telecommunications operators were given approval for 5G services.
It is continuing to monitor 5G base stations around all French airports in co-operation with the agency responsible for radio frequencies.
