Switzerland's economy minister said on Friday she believed parliament would back a crucial deal to pass on data of U.S. clients of UBS to U.S. tax authorities.

Asked whether she was optimistic parliament would approve the UBS deal, Doris Leuthard said: Yes, I think there's movement toward a majority.

Berne agreed in August to hand over to U.S. tax officials the names of 4,450 tax evaders holding secret accounts with UBS, piercing a hole in the country's famed bank secrecy laws.

But a Swiss administrative court ruling in January blocked the data transfer, forcing the Swiss government to tweak the settlement, which needs parliament approval to become effective.

The ultra-nationalist SVP, the country's largest party, has already said it will vote against the UBS tax deal. It could kill the deal if joined by the Social Democrats, the country's second-largest political force.

Leuthard, who is also serving as Swiss president this year, said her government had no back-up plan if the UBS deal, which currently faces some political opposition in Switzerland, was rejected.

There's no plan B. We want to stick to our word. We think this deal is necessary, important, she said.

Aiming to appease the deal's opponents, the Swiss government on April 28 proposed measures to make bankers' bonuses less tax-friendly.

The Social Democrats, who could make or break the parliamentary vote in June, said the proposed measures, to apply to banks and insurers, were a step in the right direction but did not go far enough.

Among the measures, bonuses linked to a company's performance would no longer count as personnel expenses, but would instead be treated as taxable corporate profit.

(Editing by Jon Loades-Carter)