Swiss bank UBS, charged with tax fraud in Paris, has been ordered by French officials to post €1.1bn in bail after being placed under formal investigation for allegedly helping rich French clients stash money in Switzerland.

It was not immediately clear if any individuals at Switzerland's largest bank were named as the subjects of the bail order, issued in Paris on 23 July.

UBS said it will appeal the bail order.

The bank, already under investigation for unlawful solicitation of customers in France, put out an unusually strongly worded condemnation of the bail order. The supposed sales practices being probed involved, seeking out wealthy French customers interested in opening bank accounts inaccessible to French tax authorities.

"We were notified today at a convocation hearing in Paris of an unprecedented and unwarranted amount of bail amounting to €1.1bn in the ongoing investigation of UBS AG's French cross-border case," the Zurich-based bank told Reuters on 23 July.

"We consider both the legal basis for the bail amount and the method of calculation to be deeply flawed and will appeal ... It is not acceptable to us that this has become a highly politicised process."

"We will continue to defend our case strongly," UBS said.

"In the course of the last few years, we have done everything we can to bring this matter to a close. We have also taken significant and broad steps to ensure tax compliance of our clients and will continue to do so," UBS added.

Three former and current executives with UBS' French subsidiary have also been individually placed under investigation, a spokesman at the subsidiary said on 23 July.

In 2013, a French prosecutor put the UBS head office and its subsidiary in France under formal investigation on charges of illegal sales practices and complicity in illegal sales practices, respectively.

French judicial authorities investigating UBS visited the bank's Paris headquarters in November 2013.