PARIS - Orange's boss Stephane Richard is under pressure from the French government after a major glitch on the telecoms firm's network prevented calls to emergency services for several hours, putting lives at risk.

Interior Minister Gérald Darmanin summoned Richard early on Thursday and called the incident "serious and unacceptable". He said one person in Brittany may have died because emergency services were not called quickly enough.

Darmanin had to cut short a trip to Tunisia and return to France to hold emergency meetings and a news conference.

The minister for digital affairs, Cedric O, said the state -- Orange's top shareholder -- will launch an investigation into the causes of the glitch. He said it affected the whole territory.

Orange's shares were down by 0.7% at 1030 GMT. 

The network outage sporadically impacted calls to emergency services in several French regions from Wednesday afternoon, Orange said early on Thursday.

French authorities hastily put place alternative emergency phone numbers that they publicized on social media on Wednesday night.

There has been a "significant improvement" of the situation since midnight, Orange said.

Orange hasn't yet explained in detail the cause of the incident, saying that a "technical equipment" had heavily disturbed emergency calls, all of which are centralized and dispatched by the former monopoly.

Orange's French rivals SFR and Bouygues Telecom said on Twitter that some of the calls to fixed lines, beyond the ones made to emergency calls, had been affected by the outage.

François Braun, the president of Samu-Urgences, the central organization for emergency medical services in France, told news channel BFM TV that a third of the calls made to his services failed.

The incident originated from a failure of a core element of Orange's network that itself stemmed from calls made via Internet boxes, the leader of CFE-CGC union at Orange, Sebastien Crozier, told Reuters.

This affected equipment centralizes all emergency calls, whether they're made by mobile phones, fixed lines that rely on the old copper network or internet boxes. It then dispatches them to emergency services, including geopositioning data, Crozier said.

Orange wasn't immediately available to confirm this information.

Richard apologised to the people who were affected by the outage on Twitter after his meeting with the interior minister.

The network outage comes a day after a French prosecutor told judges of the Paris appeals court that he should face three years in jail for his alleged involvement in a disputed compensation payment made by the state in 2008. 

(Reporting by Mathieu Rosemain, Sudip Kar-Gupta and Matthieu Protard; Editing by Jason Neely, Alexander Smith and Christina Fincher) ((sudip.kargupta@thomsonreuters.com; +33 1 49 49 53 84;))