Salah Abdeslam has revealed he wanted to blow himself up but then changed his mind, Paris prosecutor Francois Molins says.
The 26-year-old Paris attacks suspect has been charged with terrorism offences in Belgium a day after he was seized in a dramatic raid.
Salah Abdeslam will fight extradition to France but has been co-operating with police, his lawyer says.
The Paris attacks on November 13 left 130 people dead and dozens injured.
ISIS said it was behind the bombings and shootings.
Salah Abdeslam is charged with participation in terrorist murder and the activities of a terrorist group, Belgium’s federal prosecutor’s office says.
Paris prosecutor Francois Molins told a news conference: “Salah Abdeslam today during questioning by [Belgian] investigators affirmed that, and I quote, <<he wanted to blow himself up at the Stade de France and that he had backed down>>.”
Salah Abdeslam’s assertions should be treated with caution, the prosecutor added.
The French national, born in Belgium, is in custody following his arrest in Brussels on March 18 after four months on the run.
Investigators hope Salah Abdeslam, who was shot in the leg during his arrest, will reveal more information about the ISIS network behind the Paris attacks, its financing and plans.
They believe he helped with logistics, including renting rooms and driving suicide bombers to the Stade de France.
Salah Abdeslam is believed to have fled shortly after the attacks, returning to the Molenbeek district of Brussels.
Interpol has meanwhile urged “extra vigilance” at borders following yesterday’s raid in Brussels, saying more accomplices may try to flee Europe.
The subject of a massive manhunt, Salah Abdeslam was arrested about 1,600ft from his home in Molenbeek. His brother, Brahim, was one of the Paris attackers, who blew himself up.
Another man arrested at the same time as Salah Abdeslam on March 18, Monir Ahmed Alaaj, has also been charged with participation in terrorist murder and the activities of a terrorist group, the Belgian prosecutors say.
Yesterday’s raid also saw three members of a family detained.
They include Abid Aberkan, described as a friend of Abdeslam, who has been charged with participation in the activities of a terrorist organization and harboring criminals.
Another family member, identified as Djemila M, has been charged with harboring criminals, but is not in custody, the prosecutor’s office says.
Abid Aberkan’s mother, Sihane, has been freed and faces no charges.
The raid came after Salah Abdeslam’s fingerprints were found in a flat in another Brussels district, Forest, raided on March 15.
Dramatic footage showed Salah Abdeslam being bundled into a police car on March 18 after a volley of gunfire. Monir Ahmed Alaaj was also injured during the arrests, but both suspects were discharged from hospital on March 19.
France’s President Francois Hollande said Salah Abdeslam’s arrest was “an important moment”.
Prosecutors said Monir Ahmed Alaaj had travelled with Salah Abdeslam to Germany last October, where his fingerprints were taken during an identity check.
A false Syrian passport in Monir Ahmed Alaaj’s name and Belgian identity papers under an alias were found in a flat in Forest raided on March 15.
More details have meanwhile emerged about an Algerian national, Mohamed Belkaid, shot dead in March 15 raid.
Associated Press says it has been passed documents by the Syrian opposition news site, Zaman al-Wasl, suggesting he joined ISIS in 2014 and asked to be a suicide bomber.
Belgian investigators say he is likely to have been an associate of Salah Abdeslam.
Along with the Stade de France, the Paris bombings and shootings targeted the Bataclan concert hall and bars and restaurants.
Officials have identified most of the people they believe to have carried out the assaults and on March 18 Francois Hollande predicted that more arrests could follow.
Most of the suspects either died during the attacks or were killed in subsequent police raids.