Human rights groups and the country's data protection authority have expressed concern about users' privacy being compromised, with national data watchdog the Commission Nationale de l'Informatique et des Libertes, or CNIL, saying that while it recognized the government's aims were legitimate, personal freedom could be at risk.

But Minister of Public Action and Accounts Gerald Darmanin welcomed the news on Twitter, saying: "The constitutional court has just ruled that this experiment conforms to the constitution … one more tool to fight fraud!"

He also told Le Figaro newspaper "If you say you're not a fiscal resident in France and you keep posting pictures on Instagram from France, there might be an issue."

Sites such as Facebook and Instagram, as well as online selling platforms such as eBay, could be monitored for clues about additional sources of income.

"I'd like to point out that there is nothing extraordinary here, other countries are already doing it, such as the United States or Britain since 2010 for example," Darmanin told the Reuters news agency.

When the changes were first proposed, CNIL said that such a far-reaching data management operation could "significantly change individuals' behavior online, where they might not feel able to express themselves freely on the platforms in question".

The court approved the legislation as part of a wider range of tax law changes by the French government, which is carrying out a three-year experiment in increased online monitoring, but has made it subject to certain limitations.

Anything that is password-protected is off limits to the authorities, and any information gathered can only be used in relation to the person who has shared it.

But free speech campaigners are still far from satisfied, saying the lack of precision regarding the move could be the start of something more far-reaching.

"An experiment without any goals is a joke," said Arthur Messaud, from French internet freedom advocacy group La Quadrature du Net.

"We're putting the cat among the pigeons by allowing the generalized monitoring of the Internet for everything and anything."