Belgium is seeking to retrieve about €540m (£390, $600m) in unpaid tax after its probe into 829 accounts in HSBC Private Suisse Bank in Geneva.
Belgian finance minister Johan Van Overtveldt earlier made the estimate, which has been confirmed by various media outlets.
The people, who were found to be holding cash illegally in Swiss accounts, include diamond merchants, lawyers, aristocrats, company heads and sports personalities, according to local media reports.
The Belgian justice ministry, which charged HSBC's Swiss unit for serious organised tax fraud, is carrying out a separate investigation into the illegal accounts and may prosecute the wrongdoers.
An international collaboration of news outlets, including the Guardian, the French daily Le Monde, BBC Panorama and the Washington-based International Consortium of Investigative Journalists, citing leaked secret bank account files, revealed that the bank's Swiss arm helped wealthy customers across the globe dodge taxes and hide millions of dollars worth of assets.
The news outlets obtained the data from Herve Falciani, a former IT employee of HSBC's Swiss private bank, who was charged in Switzerland with industrial espionage and breaching the country's secrecy laws.
France, Argentina and Switzerland have indicted HSBC, in addition to probing its account holders.
So far, the UK has recovered £135m from HSBC clients and France £188m.