Bondholders of Credit Suisse Group AG have reportedly filed a lawsuit against Switzerland's banking regulator. The value of the additional Tier 1 bonds they hold accounts for $4.5 billion of the $17 billion in bonds the company has liquidated.

The indictment alleges that the Swiss Financial Market Supervisory Authority, Finma, acted unconstitutionally when it ordered Credit Suisse to cancel the additional Tier 1 bond debt.

The move by the Swiss regulator in mid-March angered bondholders, who had thought they would be better protected than shareholders in the rescue deal Credit Suisse struck with UBS earlier that month.

The lawsuit was filed by law firm Quinn Emanuel on Wednesday in the city of St. Gallen in eastern Switzerland.