Belgium on Friday charged Swiss banking giant UBS with "serious and organised" fiscal fraud for encouraging clients to cheat on their taxes, as well as being involved in money laundering.

UBS is also under investigation in the US and France accused of using Switzerland's banking secrecy laws to help rich clients avoid tax in their home countries.

"The Swiss bank is suspected of having approached Belgian clients directly (without going through its Belgian unit) with the aim of getting them to sign up to tax evasion products," the federal prosecutor's office said in a statement.

It said the charge sheet covered criminal activity involving "money laundering, illegally acting as a financial intermediary in Belgium, and serious and organised fiscal fraud."

A spokeswoman for the prosecutor's office said there was no deadline to close the case, and a judge would decide to hold a trial or not.

The charges followed "excellent cooperation" with the French authorities, the statement said, adding that no further details would be released for the moment.

UBS told AFP in an email that it would "continue with its vigorous defense against unfounded allegations."

In 2014, the head of the UBS subsidiary in Belgium, Marcel Bruehwiler, a Swiss national, was also indicted for money laundering and tax fraud.

At the time, Belgian officials spoke of billions of euros (dollars) deposited in secret bank accounts in Switzerland on behalf of rich Belgians.

Last week, French judges completed an investigation into allegations that UBS and its local unit had encouraged clients to open accounts in Switzerland so as to avoid the taxes during the period 2004-12.

UBS has been embroiled in a whole series of similar cases, most notably in the United States where the authorities said the bank allowed US customers to conceal their assets and income from the taxman.