France’s data protection watchdog has urged caution over plans to allow authorities to monitor individuals’ social media posts and purchasing activity on websites such as eBay in order to identify those committing tax fraud.

The French parliament is to debate proposals for a three-year trial during which the tax office’s computer system would collect information on peoples’ lifestyles from social media accounts such as Facebook, Instagram and Twitter, and also monitor their activity on sites such as eBay and the French site Le Bon Coin.

The idea, first suggested by the public action and accounts minister, Gérald Darmanin, is aimed at identifying gaps between a person’s declared revenue and their lifestyle, as evidenced on their social media accounts.

Darmanin told French TV last year that the tax office “will be able to see that if you have numerous pictures of yourself with a luxury car while you don’t have the means to own one, then maybe your cousin or your girlfriend has lent it to you, or maybe not”.

It could also potentially detect customs fraud on online sales sites.

The system would also be able to check where a person is mainly residing, for example if they have declared themselves domiciled abroad but are spending most of their time in France.

The data watchdog la Commission Nationale de l’Informatique et des Libertés (CNIL) has given written advice that the government must show “a great deal of prudence” over the plan, which raised “unprecedented questions over personal data protection”. It said the plans could amount to the mass collection of data on a “significant scale”, with information being automatically mined rather than gathered for specific, suspicious cases.

The watchdog said such a massive collection of data could “significantly change individuals’ behaviour online, where they might not feel able to express themselves freely on the platforms in question”.

The French government has suggested the trial would only run for three years.

• This article was amended on 2 October 2019 to remove an incorrect reference to HMRC’s computer system.