Credit Suisse investors who held a total of more than 4.5 billion Swiss francs ($5 billion) of the bank's Additional Tier 1 (AT1) bonds have sued the Swiss banking regulator, challenging the legality of writing them down.

AT1 bonds totalling 16 billion Swiss francs were wiped out last month during the state-backed rescue of Credit Suisse by its Swiss rival UBS, angering bondholders who thought they would be better protected than shareholders.

The lawsuit, filed on April 18 by Quinn Emanuel Urquhart & Sullivan, challenges FINMA's decision leading to the write-down, the law firm representing the bondholders said on Friday.

"FINMA's decision undermines international confidence in the legal certainty and reliability of the Swiss financial center," Thomas Werlen, the firm's managing partner in Switzerland, said in a statement.

FINMA declined to comment on the lawsuit. Credit Suisse did not immediately respond to a Reuters request for comment.

($1 = 0.8941 Swiss francs)

(Reporting by Jahnavi Nidumolu and Jyoti Narayan in Bengaluru; Editing by Savio D'Souza and Alexander Smith)