Telegram boss and founder Pavel Durov has been placed under formal investigation in France as part of a probe into organised crime on the messaging app, Paris prosecutors say.

Mr Durov, 39, has not been remanded in custody, but placed under judicial supervision, and has to pay a €5m (£4.2m; $5.6m) deposit.

The Russian-born billionaire, who is also a French national, also has to show up at a French police station two times a week and is not allowed to leave French territory.

He was first detained upon arrival at Le Bourget airport north of Paris last Saturday under a warrant for offences related to the app.

In Wednesday's statement, the Paris prosecutors said Mr Durov was put under formal investigation over alleged offences that included:

• Complicity in the administration of an online platform to enable illicit transactions by an organised gang
• Refusal to communicate with authorities
• Complicity in organised criminal distribution of sexual images of children

In France, being put under formal investigation does not imply guilt or necessarily result in a trial - but it indicates that judges consider there is enough of a case to proceed with an investigation.

Mr Durov has so far made no public comments on the latest developments.

On Monday evening, the Paris prosecutors said Mr Durov was being held in custody as part of a cyber-criminality investigation. In response, Telegram said Mr Durov had "nothing to hide".

French President Emmanuel Macron said this week the decision to hold Mr Durov was "in no way... political".

However, Russia said that without a "serious basis of evidence", the charges could be seen as an act of "intimidation" against a major technology company for political purposes.

Telegram - which offers end-to-end encryption - is ranked as one of the major social media platforms.

It was founded in 2013 and is particularly popular in Russia, Ukraine and other former Soviet Union states, as well as Iran.

The BBC revealed on Wednesday that Telegram - which has more than 950 million registered users - has repeatedly refused to join international programmes aimed at detecting and removing child abuse material online.

The BBC has contacted Telegram for comment about its refusal to join the child protection schemes.

Mr Durov, who also founded the popular Russian social media company VKontakte, left Russia in 2014 after refusing to comply with government demands to shut down opposition communities on the platform.

He also holds passports from St Kitts and Nevis and the United Arab Emirates.

Telegram's main office is in Dubai.