Valerie Trierweiler, the long-term girlfriend of French President Francois Hollande, was to remain in hospital on Monday where she was admitted after reports emerged her partner was having an affair with an actress.
A photo taken on September 3, 2013 shows French President Francois Hollande and his partner Valerie Trierweiler at the Elysee Palace in Paris
Trierweiler, 48, was taken to hospital suffering from stress on Friday, and was initially expected to check out on Monday.
But "doctors believe she needs more rest," an aide to Trierweiler said.
Her extended hospital stay will likely loom large over a high-profile press conference by Hollande on Tuesday, when he is to address about 500 journalists to set out his policy plans for the new year.
The Socialist leader, 59, had initially been hoping to ride out the storm created by last week's revelation of an affair with Julie Gayet, a blonde actress 18 years his junior.
But those hopes were shattered with the hospitalisation of Trierweiler, Hollande's companion of several years and de facto First Lady.
Symptoms variously described in the media as low blood pressure, exhaustion and a "severe case of the blues" developed within hours of glossy French magazine Closer publishing details of Hollande's alleged secret trysts with Gayet in a borrowed apartment close to his official residence, the Elysee Palace.
Since then Hollande has faced mounting pressure to clarify the position of Trierweiler, who lives with him at the Elysee Palace.
"He has to clarify the situation," said Thierry Mandon, the spokesman for the Socialist Party's parliamentary group.
"He really must clear up his personal situation so that we can address the serious issues we face. He has to do it once, firmly and decisively, and then we don't talk about it any more."
The traditional reticence in France's media and political class over what is seen as prying into the private lives of public figures ensured that reaction to Closer's scoop was at first subdued.
Despite concerns that Hollande had apparently been taking risks with his own security with clandestine visits to the flat on a chauffeur-driven scooter, it looked like he would be allowed to resolve his personal dilemma behind closed doors.
But the fact that Trierweiler is effectively a public figure with an entourage funded by the taxpayer has made her future a legitimate news story.
Hollande himself has slammed Closer's report as an outrageous attack on his right to a private life but has not denied the substance of the magazine's claims.
Left-wing daily Liberation said that Trierweiler's hospitalisation meant the affair could no longer be brushed off as a purely private matter.
"It is hard to see how the president can avoid clarifying the situation -- both medical and official -- of the First Lady," Liberation commented.
Closer meanwhile announced that it would be printing an extra 150,000 copies of the edition carrying its scoop in response to demand that has seen copies change hands on the Internet for up to 10 times the regular cover price.
Having put her career as a journalist for Paris Match on hold after Hollande's 2012 election, Trierweiler has accompanied the president on overseas visits to China, Japan and India and is due to travel with him to Washington next month.
Rumours that Hollande had become enamoured of Gayet have swirled around Paris since the actress appeared in a campaign video for him in which she gushed about him being "fantastic, humble and really ready to listen".
In March 2013, she filed a legal complaint for intrusion on her privacy after suggestions of an affair with Hollande were published on a number of obscure websites and blogs.
That helped to fuel speculation that all was not well at the Elysee, but friends of the First Lady quoted by Le Parisien newspaper have insisted she was devastated when the report of the affair emerged.
"She is ready to forgive him, she doesn't want to slam the door but she has to know quickly what his intentions are," the local paper quoted a friend as saying.
Opinion polls suggest French voters are also willing to forgive Hollande his alleged infidelity: a weekend survey found that more than three quarters (77 percent) think his love life is his own business -- though that was before Trierweiler's hospitalisation.
Twice-divorced Trierweiler has officially been Hollande's partner since 2007, when he left Segolene Royal, a fellow heavyweight in the Socialist Party and the mother of his four children.
Gayet, meanwhile, has not spoken publicly about the affair allegations, but her ex-husband on Monday said she was remaining "very calm".
"It's obviously not easy. At the same time, she's very calm about it all and very sure of herself, I think, because there was no fault, no betrayal," the Argentinian writer Santiago Amigorena said on Europe 1 radio.