Political opponents could block a compromise between Switzerland and US tax authorities that would settle a dispute on untaxed money in Swiss accounts once and for all, newspapers reported on Sunday.
Switzerland's NZZ am Sonntag said confidential documents showed a one-off payment would allow Swiss banks to get rid of claims, concerning past wrongdoing, in the United States.
But Switzerland's biggest party, the right-wing People's Party SVP, as well as the Free Democrats FDP, were likely to reject an addition to the double taxation agreement with the US that would allow information requests based on behavioural patterns.
Parliament is due to vote on this over the next days.
Switzerland has in the past ruled out "fishing expeditions", considering they infringed its bank secrecy law.
Urs Schwaller of the Christian Democrats CVP told the newspaper that banks needed to acknowledge they had broken US law and the government needed to demonstrate that a definite solution to all US tax problems was within reach.
"If these conditions are met, the negotiated compromise is acceptable," Schwaller said.
Media reported on Saturday, Switzerland's top tax negotiator Michael Ambuehl had negotiated a compromise that would allow the 10 banks currently investigated by US authorities to get off the hook by paying a fine and handing over client data.
The compromise could also prevent future legal problems in the US for other banks.
Swiss newspaper Der Sonntag said on Sunday the compromise would include the transfer of data of thousands of clients to US authorities, citing Zurich lawyer Milan Patel, who used to work for US tax authorities.
Patel said an extension of the investigation to other banks was likely. "The US are going to proceed step by step in the investigation against those who helped tax dodgers. There's a big risk that the list of 10 banks will be extended," he said.