The ruling was issued by the Federal Administrative Court, Finma said in a statement on its website.

Finma, the Swiss financial regulator, told the Zurich-listed banking and wealth management firm to surrender client data comprising 250 accounts to the US authorities, as part of a criminal case in which UBS admitted helping wealthy US clients evade taxes. At the time, UBS agreed to pay a $780 million fine to settle criminal charges.

The case – and a separate civil case settled in August – threatened to cause a serious breach to Switzerland’s historic bank secrecy laws, which have helped make the Alpine state one of the world’s richest financial centres. Financial services account for about 13 per cent of Swiss gross domestic product.

UBS’s legal wrangles in the US came at a time when countries such as Switzerland were under mounting pressure from other governments to be more co-operative in hunting for tax cheats. Tax evasion is not a crime in Switzerland – unlike the position in countries such as the US and UK.

Today’s court ruling may derail last February’s agreement, although it is unclear whether any client accounts being transferred will now be halted because of the ruling. UBS declined to comment on the matter when contacted by this publication. Finma did not immediately return calls.

In its defence, Finma said that it based its decision on Articles 25 and 26 of the Swiss Banking Act, which give it the authority and obligation to impose unspecified preventive measures if it has reasonable grounds to suspect serious liquidity problems.

Finma said it will study the ruling before deciding whether to launch an appeal.