The former NatWest chief executive breached data protection laws when she spoke to a BBC journalist about the planned closure of Nigel Farage’s bank accounts, the UK’s information watchdog has ruled.

An Information Commissioner’s Office (ICO) report seen by the Guardian said that Alison Rose broke rules on two counts: first by revealing that Farage had a banking relationship with its private bank, Coutts; and secondly by providing “misleading information” that led the BBC to believe the bank was closing his accounts for purely commercial reasons, linked to his wealth.

“Mr Farage’s rights were infringed because of this,” the ICO report said.

Documents obtained by Farage in July showed that while Coutts had considered the fact that he had fallen below the bank’s multimillion-pound account thresholds, the bank also decided to close his accounts due to concerns over his political views, which it said did not align with the bank’s. The Coutts documents showed the bank believed that his alleged “xenophobic, chauvinistic and racist views” posed a reputational risk to the bank.

ICO said it did not plan to take any further action, given that NatWest – which is 38.5% owned by the taxpayer – had already launched an investigation into the incident and Rose had stepped down over the scandal.

In July, Rose admitted to discussing the matter with a BBC journalist, in an alleged breach of client confidentiality that raised concerns in government and eventually led to her resignation.

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.”

The ICO ruling, which was first reported by the Financial Times, was issued in response to complaints filed by Farage this summer. It also comes days before NatWest’s own investigation is due to conclude, at the end of October.

That investigation, which is being conducted by the law firm Travers Smith, could result in Rose losing millions of pounds in pay. NatWest confirmed last month that she is still likely to collect £2.4m as she serves out her 12-month notice period.

Farage said: “The ICO report confirms that Dame Alison Rose was in breach of data rules, of the FCA rulebook, and oversaw a culture of deep political prejudice within the NatWest Group. She must not be rewarded for failure.”

A spokesperson for Farage told the Guardian that the former Ukip leader had not ruled out taking the matter to court.

NatWest bosses, including the outgoing chair, Howard Davies, are likely to face pressure over the ICO ruling when the bank releases its third-quarter earnings on Friday morning.

A NatWest spokesperson said: “We fully cooperate with the ICO in its assessment of any customer complaint, but it would not be appropriate for us to comment on this individual case.”

A spokesperson for Rose said: “Alison Rose was not aware of the ICO investigation, did not receive any questions from the ICO, has not seen its ruling and cannot comment on it.”