The investigation, headed by Judge Michel Claise, began late last year.
Its announcement comes after claims in Belgique magazine that UBS had approached wealthy clients and encouraged them to funnel money into undeclared accounts in Switzerland.
“We have not been informed of any preliminary investigation concerning UBS Belgium,” said a spokesperson for the bank’ wealth management arm.
“UBS conducts its business in full compliance with applicable laws and regulations.”
“UBS does not tolerate any activities intended to help its clients circumvent their tax obligations.”
The claims follow a string of cases around improper behaviour by Swiss banks including tax evasion claims by the US authorities, levelled against UBS and giant rival Credit Suisse.
UBS has also been fined by authorities in the US, UK and Switzerland over allegations its traders attempted to manipulate interest rate benchmark Libor.