French government officials have been ordered not to use handheld Blackberry devices amid fears that foreigners could spy on them, reports say.
Blackberry's makers deny that its system is not secure
Workers in the French president's and prime minister's office have been told their e-mails risk falling into foreign hands, Le Monde newspaper reports.
France's SGDN security service is worried because Blackberries use US- and UK-based servers, the paper says.
But some officials are flouting the ban and using them in secret, it adds.
"They tried to offer us something else to replace our Blackberries but it doesn't work," one unnamed official told the paper.
More than seven million people around the world now use the Blackberry, which is made by Canadian firm Research In Motion (RIM).
A member of France's governing UMP party in the National Assembly, Jacques Myard, said French politicians needed to be aware that Blackberries were not secure devices.
"It's very good to say 'be careful', because we don't live in an ordinary world in which you can talk on your mobile or private line without any warning, without caution," he told the BBC's World Today programme.
RIM responded to the allegations by saying that Blackberry's encryption system was "the most secure wireless data solution available", pointing out that it had been approved for the transmission of sensitive data by the UK government and by Nato.
The US, Australia, New Zealand, Austria and Canada had also given the system their accreditation, the company added.
RIM said the US National Security Agency did not have the ability to view the content of any data communication sent through Blackberry servers.
"RIM continues to be committed to working with and supporting the needs of both corporate and government customers within France, including protecting data from attack and unauthorised access," it added.