By Dominique Vidalon and Elizabeth Pineau

PARIS (Reuters) -French investigative judges are due to decide on Wednesday whether to place Russian-born Telegram boss Pavel Durov under formal investigation following his arrest as part of a probe into organised crime on the messaging app.

Durov's detention as he landed at an airport near Paris on a private jet on Saturday evening has put the spotlight on the criminal liability of app providers and fuelled debate on where freedom of speech ends and enforcement of the law begins.

The judges' decision is expected by 8.00 p.m.(1800 GMT), 96 hours - or four days - after Durov was taken into custody, the maximum period he can be detained before they decide whether or not to put him under formal investigation.

Durov's arrest has also put in focus the uneasy relationship between Telegram, which has close to 1 billion users, and governments.

President Emmanuel Macron, who with his team uses Telegram to communicate, had lunch with Durov in 2018 as part of a series of meetings the French leader had with tech entrepreneurs, a source close to Macron said.

Being placed under formal investigation in France does not imply guilt or necessarily lead to trial, but indicates that judges consider there is enough to the case to proceed with the probe. Investigations can last years before being sent to trial or shelved.

If Durov, who has been in police custody since his arrest, is placed under formal investigation, judges will also decide whether to put him in pretrial detention. One of the factors they will consider is whether he could try to flee.

A source at the Paris prosecutor's office said an update on the probe was likely to be issued late on Wednesday.

INVESTIGATION

The overall investigation is at this stage directed against unspecified people.

It focuses on suspected complicity in crimes including running an online platform that allows illicit transactions; possessing images of child sex abuse; drug trafficking; fraud; refusing to pass information to authorities; and providing cryptographic services to criminals, prosecutors said.

The prosecutor's office did not say which crime or crimes Durov himself might be suspected of.

Durov's French lawyer did not reply to repeated Reuters requests for comment through emails and phone calls.

Faced with accusations from Russia, and also Elon Musk, over the stifling of freedom of speech with Durov's arrest, Macron took the unusual step on Monday of issuing a message on X about what he said were "false information."

France, he said, was committed to free speech and the independence of the judiciary, which he said had decided alone to arrest Durov.

A source close to the matter reiterated on Wednesday that Macron and his government had nothing to do with the arrest.

The foreign ministry did not immediately respond to a request for comment on a Wall Street Journal report saying French and United Arab Emirates authorities jointly hacked Durov's phone in 2017. Durov has French citizenship since 2021.

(Additional reporting by Tassilo Hummel, Writing by Ingrid Melander, Editing by Kevin Liffey and Timothy Heritage)