Switzerland is trying to ensure UBS is not forced to hand over account details to authorities in the United States.
In an apparent reversal of promises to share tax data, the Swiss government is arguing that handing over client details would breach its national bank secrecy rules.
UBS is refusing to release data on 52,000 Americans who hold Swiss bank accounts to US tax authorities who accuse them of tax evasion.
UBS faces a court hearing in Miami on 13 July. It is Switzerland's largest bank.
The BBC reports UBS is accused of hiding almost $US15 billion in assets in secret accounts.
In February, UBS handed over the details of 250 US clients to American authorities to avoid a criminal case, but it is continuing to refuse to pass on data on the further 52,000.
No more information
On Wednesday, Switzerland's Justice Ministry said UBS cannot hand over any more information about clients without breaking Swiss rules.
Earlier this year, Switzerland pledged to meet global standards on sharing bank information, and establish reciprocal arrangements with other governments including the US.
However, the government had repeatedly said it will hand over information only "in individual cases where a specific and justified request has been made", or when requests are "concrete and justified".