ZURICH (Reuters) - Swiss bank UBS <UBSG.S> has failed to persuade the European Court of Human Rights to overturn an order that it must post 1.1 billion euros ($1.17 billion) bail in a French case about whether it helped clients avoid tax.

Switzerland's biggest bank took the unusual step of going to the human rights court in 2015 after French courts upheld an order by investigating judges that the bank must put up the bond until the legal case was resolved.

The Strasbourg court said on Thursday it had unanimously rejected UBS's application to challenge the order.

"The Court held that the security required constituted an interim measure which did not prejudge the outcome of the proceedings and that the amount had been assessed by the domestic judges," the ECHR said in a statement, adding that the decision is final.

French investigators have sought a fine of up to 4.88 billion euros, the ECHR said.

The France case is one of the last major legal issues facing UBS, which in recent years has paid out billions in fines to settle cases over tax evasion and benchmark manipulation.

A source close to the matter confirmed to Reuters a report by Les Echos earlier this month that the bank and prosecutors were negotiating a settlement under a new French law which would allow UBS to avoid a trial.

UBS said in a statement it would "continue to strive for a solution to the proceedings in France while strongly defending our position".

"The facts do not support the magnitude of the bail and we have a fiduciary responsibility to protect both the financial and reputational interests of our stakeholders. We regret the court's decision and disagree with its reasoning," it said.

(Reporting by Joshua Franklin and Angelika Gruber in Zurich, and Chine Labbé in Paris; Editing by Adrian Croft)