Paris attacks suspect Salah Abdeslam posted the flag of the so-called Islamic State (IS) group on his Facebook page three weeks before the attacks, Belgian website RTBF reports.
Authorities were aware of the flag but did not act upon it, it added.
IS militants staged bomb and gun attacks on several targets in Paris on 13 November, murdering 130 people and seriously wounding many others.
Salah Abdeslam was captured in Brussels and extradited to France last month.
He was on the run for four months before he was arrested four days before the Brussels bombings and has been linked to several of the 22 March attackers. He is now held in a high-security jail just outside Paris while being investigated over charges of terrorism-related murder and attempted murder.
Belgian authorities have been accused of failing to establish links between him and his brother, Brahim Abdeslam, to ringleader Abdelhamid Abaaoud. Brahim Abdeslam blew himself up on 13 November while Abaaoud was killed during a police raid in Paris days afterwards.
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Salah Abdeslam posted the IS flag on his own page on 23 October 2015, three weeks before the jihadist attacks in Paris, according to RTBF, Belgium’s French-language public broadcaster.
While it did not escape the attention of Belgium’s Co-ordinating Unit for Threat Analysis (OCAM), there was no response from the police or intelligence services, the report says.
Belgian authorities have already admitted to key blunders in the months leading up to the Paris and Brussels attacks.
But further failings involving Salah Abdeslam have emerged. RTBF said on Monday that an informer had come forward from the Brussels area of Molenbeek, where the Abdeslam brothers lived, as early as January 2015.
According to the informant, Salah Abdeslam had been in touch with Abaaoud after the IS attacks on Paris between 7 and 9 January 2015, and even provided his mobile number.
On 15 January, Belgian police broke up a jihadist cell in Verviers, apparently run by Abaaoud, but he was living in Athens at the time and evaded Greek authorities.
It has already been widely reported that at least one of the Abdeslam brothers had been radicalised as early as 2014. By February 2015, both men had a police file but were not treated as high priority because of the large number of Belgian jihadists returning from Syria.