Thomson Reuters
The logo of UBS is seen outside the building housing the headquarters of the Swiss bank in San Juan
The bank said it "strongly" denies the allegations of "interfering with the witness" and is "confident that we can vigorously defend our rights and position."
It's been a particularly bad month for UBS, which is struggling with anti-money-laundering and tax evasion probes.
The allegation comes as French prosecutors wrap up a probe into whether the bank helped clients evade taxes and just a few days after the bank was accused of money laundering in Belgium.
According to a Bloomberg report last week, Belgian authorities said UBS Group AG committed money laundering and fiscal fraud offences when helping clients evade taxes. The prosecutors said they had an "excellent collaboration" with authorities in France. UBS called the allegations unfounded.