LONDON — Five unnamed U.K. banks are being forced to make major changes to the ways they deal with customer complaints, and two of them face possible fines, after the U.K. Financial Services Authority found evidence of "unacceptable standards" in the way some banks handle complaints.

Royal Bank of Scotland Group PLC in February had disclosed it was under investigation by the FSA for complaint handling. A spokesman Wednesday said the bank is continuing to cooperate with the FSA on the probe.

Subscribe Now

Access to authoritative analysis and perspective and our data-driven report series.

14-Day Free Trial

No credit card required. Complete access to articles, breaking news and industry data.