"The Swiss bank is suspected of having directly, and not via its Belgian subsidiary, approached Belgian clients to convince them to set up structures aimed at evading taxes," Brussels prosecutors said in a statement.

But in an emailed statement UBS said: "The Belgian authorities have confirmed today by way of a press release, that the investigating magistrate has opened a formal investigation against UBS AG. Any discussion of potential charges at this stage is premature."

Brussels prosecutors declined to comment further.

The Belgian prosecutors' announcement is the latest in a series of investigations by European and U.S. authorities into Swiss banks, where strict banking secrecy rules had made it a haven for untaxed assets.

Neighbour France placed UBS under formal examination in 2014 and the investigating judges ordered UBS to provide bail of 1.1 billion euros (£866.2 million).

Belgian prosecutors said in the statement the charges against UBS had been a result of cooperation with French authorities and the work of an inquiry committee.

In 2014, Belgian police carried out raids at the bank and at the homes of a client and of UBS Belgium Chief Executive Marcel Bruehwiler. Bruehwiler was also charged at the time.

The bank's Belgian subsidiary, which employed some 60 staff including 20 private bankers, has since been sold to Belgian private bank Puilaetco Dewaay.

(Reporting by Robert-Jan Bartunek in Brussels, additional reporting by Joshua Franklin in Zurich; editing by Philip Blenkinsop, David Evans and Susan Thomas)