Two-thirds of French teenagers spend more than two hours sitting on their phones or computers and do not get enough exercise, the country's health and safety agency warned in a statement released on Monday.
The lifestyle habits can result in health complications such as obesity, eating disorders and sleep problems, the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) said.
"Adolescence is a pivotal period during which acquired habits tend to persist or even become more pronounced in adulthood with associated effects on health," the ANSES said in a statement.
They explained that the current situation with an "abundance" of new digital technologies encourages a more sedentary lifestyle.
The information is part of a new evaluation and continuation of the agency's study on consumption and eating habits in the French population released in 2017.
The study was thus carried out before France entered a lockdown to curb the spread of COVID-19, which the agency said "accentuated the tendency towards physical inactivity".
Under France's current lockdown, people are allowed to exercise for up to one hour per day, but gyms are closed and group sports are not allowed.
The agency said a sedentary lifestyle means time is spent sitting or lying down in front of a screen for more than two hours a day. Physical inactivity is less than 60 minutes of activity per day.
At least 49% of teenagers between the ages of 11 and 17 present a very high health risk, ANSES said, characterised by up to four and a half hours of daily screen time and/or less than 20 minutes of physical activity per day.
Some 17% of teenagers spend more than four and a half hours on screen and get less than 20 minutes of exercise.
For 11-14 year olds, girls are less likely than boys to practice at least 60 minutes of physical activity per day, the agency said.
The agency recommended breaking periods of sitting by getting up and stretching and reducing screen time with physical activity.