The former chief executive of NatWest Group infringed on Nigel Farage's data protection rights when she talked to a journalist about his relationship with Coutts bank, Britain's information watchdog has ruled.
The Information Commissioner's Office said the disclosure had been "unacceptable" in a statement on Wednesday.
The former boss resigned earlier this year after admitting a "serious error of judgment" by discussing with a BBC journalist Farage's relationship with Coutts, which is owned by NatWest Group.
She said she did not share any personal financial information, but was "wrong to respond to any question raised by the BBC about this case".
But her departure came hours after the board said it had full confidence in her, meaning the bank was forced to backpedal after the statement was given.
The ruling comes at a sensitive time for NatWest, which is due to announce quarterly results on Friday. In a statement on Wednesday, an ICO spokesperson said:
"Following a thorough review of the complaint raised with us, we have concluded our investigation. 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."
Former UKIP and Brexit Party leader Farage told the Financial Times, which first reported on the ruling: "the ICO report confirms that Dame Alison Rose was in breach of data rules, of the FCA rule book and oversaw a culture of deep political prejudice at NatWest."
A NatWest spokesperson said:
"We fully co-operate with the ICO in its assessment of any customer complaint, but it would not be appropriate for us to comment on this individual case."
By Nina Lloyd, PA Political Correspondent