The platform was thrust into the spotlight after its chief Pavel Durov was arrested in France on Saturday. He has since been released on bail but cannot leave the country.
Durov is accused of failing to curb extremist and illegal content on the increasingly controversial platform, which has over 900 million followers.
Telegram had already been in the European Union's crosshairs before the French criminal investigation. Senior EU officials have described Telegram as an "issue".
But now independently of the French probe, the European Commission is investigating whether Telegram has the minimum number of users that would require it to comply with more stringent rules.
The EU's focus is on the "social network" part of the platform, which has channels with hundreds of thousands of subscribers, and not on one-to-one messaging between contacts, like on WhatsApp or Signal.
Since February a new law, known as the Digital Services Act (DSA), forces all platforms in the EU to protect online users from illegal and harmful content.
But platforms with at least 45 million monthly active users in the EU have even greater obligations, and they are regulated by the commission rather than national authorities.
The obligations include identifying the risks posed by the platforms and putting measures in place to mitigate them as well as ensuring third-party auditing.
The EU has designated 25 platforms as "very large" including Facebook, Instagram, TikTok, X and YouTube.
Telegram escaped the extra rules since it said it had 41 million users in the 27-country EU in February.
The commission has admitted to having "doubts" about Telegram's claims.
Question of numbers
Under the DSA, platforms must update their figures every six months. When the time came for an update in August, Telegram refused to give exact figures, insisting on its website that it had "significantly fewer than 45 million average monthly active recipients in the EU".
"What we are asking for is a precise figure. Telegram merely says they are sharply below the threshold," said Thomas Regnier, commission spokesperson on digital issues. The DSA stipulates that every platform gives specific figures.
Since Telegram's legal representation is based in Brussels, Belgian regulators have the power to monitor the platform -- meaning that currently the commission does not have jurisdiction over possible DSA violations. That is despite the fact Belgium faces infringement proceedings by the commission over the national authority's failure to be fully operational.
In parallel, the commission's data and science service, the Joint Research Centre, is undertaking its own work to calculate an estimate for Telegram's user numbers. If the EU finds that Telegram does have at least 45 million users, the platform will be designated as "very large".
But Regnier was keen to stress there was no link between the commission's efforts and the French probe. "It is not for the commission to comment on a national investigation under national criminal law. Let's be very clear: Criminal prosecution is not among the potential sanctions for a breach of the DSA," Regnier told AFP.
Violations under the DSA can be met with sanctions, mainly in the form of fines. Telegram has insisted the platform complies with EU rules. "Telegram abides by EU laws, including the Digital Services Act -- its moderation is within industry standards," the company said.