"We can confirm that the Threat Analysis Coordination Agency re-evaluated the threat level from four to level three," a spokesman for the crisis centre, which is part of the interior ministry, told AFP.
There were no immediate details on what basis the agency reduced the threat level, with the national security council of top government officials set to decide on the consequences at a meeting later Thursday.
The alert level was raised early Saturday after the government warned of a "serious and imminent threat" of attacks in the capital similar to the Paris atrocities that left 130 people dead on November 13.
Belgian Prime Minister Charles Michel had said on Monday the threat level would remain at its highest until November 30 but that it would be reviewed constantly.
Schools and the metro system reopened partially on Wednesday despite two suspects from the Paris attacks still being at large.
Belgian police on Tuesday charged a fifth suspect in connection with the Paris attacks after a series of raids carried out during the lockdown.