The Information Commissioner's Office (ICO) found that Alison Rose breached data rules over revelation of Nigel Farage bank details.
However, no further action will be taken by the watchdog as Rose has already stepped down from her role at the bank, which is conducting its own investigation.
The ICO said there were two privacy breaches involved in Dame Alison Rose’s disclosure to BBC News business editor Simon Jack regarding the closure of Farage’s account.
The BBC reported that Farage no longer met the financial criteria to hold an account with Coutts, which is part of the NatWest Group. Coutts requires account holders to have at least £1m in investments or borrowings, or £3m in savings.
The ICO judged Rose’s actions were “unacceptable” but no further regulatory action was needed as she already resigned.
Rose stepped down from NatWest in July after admitting she was the source behind the BBC’s inaccurate story over Farage’s bank account closure.
The BBC report alleged Brexit campaigner Nigel Farage was ditched by Coutts – part of the NatWest Group – for not being wealthy enough. Farage maintained the account closure was due to his political views.
No further action against Rose
An ICO spokesperson said: “We upheld two parts of the complaint – namely, we found that an individual employed by NatWest shared information when they should not have done, and that by doing so they infringed the complainant’s data protection rights.
“We have been clear with the bank that these actions were unacceptable and should not happen again. However, in view of the fact the individual in question resigned her post and the bank has commissioned its own investigation, we do not intend to take any further regulatory action at this time.”
NatWest is due to announce quarterly results on Friday.
