Tax officials used intrusive powers to rake through Osita Mba's personal data in attempt to prove he had spoken to the Guardian

Tax officials used intrusive investigative powers meant to catch serious criminals to try to prove that a whistleblower who uncovered a "sweetheart" deal with Goldman Sachs had spoken to the Guardian, it has emerged.

The belongings, emails, internet search records and phone calls of the HM Revenue and Customs solicitor Osita Mba and the phone records of his wife, Claudia, were examined by investigators, according to previously undisclosed documents.

The powers, which are supposed to be used to combat large-scale criminal tax frauds, were used because the tax inspectors suspected that Mba had been in contact with the Guardian's former investigations editor, David Leigh.

Leigh's telephone numbers and email addresses were cross-referenced with Mba's, but investigators found no evidence of contact, documents show.

The disclosure has prompted serious questions about HMRC's behaviour.

Cathy James, the head of the whistleblowers' charity, Public Concern at Work, said the decision to use intrusive powers to examine an employee who made claims using whistleblowing legislation was "outrageous" and "sinister".

"The actions of the HMRC in this case are very much a step in the wrong direction, more likely to result in a culture of silence with more anonymous leaking than anything else. It is a case of shoot – and silence – the messengers," she said.

Using the Public Interest Disclosure Act, Mba wrote to the National Audit Office (NAO) and two parliamentary committees in confidence in 2011 saying that the head of tax, Dave Hartnett, had "let off" Goldman Sachs from paying at least £10m in interest.

Emails show Mba's identity was disclosed to the revenue in October 2011 by the former clerk of the public accounts committee, who had sought clarification that Mba was their employee. The next day, a member of the HMRC's security staff sought to obtain access to Mba's office cabinet beneath his desk in Whitehall. "Thanks. Did you manage to get cabinet key number?" he asked a colleague.

The man also received an email containing the solicitor's private email address, his mobile number, his home telephone number and his wife's telephone details.

On 11 October 2011, the Guardian published a story under the headline "Goldman Sachs let off paying £10m interest on failed tax avoidance scheme", written by Leigh. Publication prompted members of the revenue's criminal investigative unit to take action. One named internal criminal investigator sent an email on 19 October to a colleague saying that the revenue had begun "a review of the suspect's [Mba's] H drive [the hard drive used within HMRC] and email traffic and internet usage", but inquiries had revealed nothing.

He then proposed a "further interrogation of computer material" and an "itemised billing check", and wrote that "consultations with the CPS [Crown Prosecution Service] can proceed".

Using the Regulation of Investigatory Powers Act 2000 (Ripa), HMRC can see websites viewed by taxpayers, where a mobile phone call was made or received, and the date and time of emails, texts and phone calls. According to the revenue website, these powers "can only be used when investigating serious crime". But the papers disclose that applications were granted to investigate Mba using Ripa.

On 21 October 2011, tax officials applied for an itemised billing request to check a mobile of Mba's, documents show.

One document read: "David Leigh, who was given HMRC material discussing a named taxpayers tax affairs advised a senior employee of HMRC that he had been given access to that material on the 4th or 5th October 2011 and in it he quoted extracts from an HMRC minute of 8/12/2010. He was clearly given information which if provided by an HMRC employee was in contravention of CRCA [Commissioners for Revenue and Customs Act 2005]."

Ten days later, another investigator sent a document, entitled leakupdate4, to colleagues showing they had failed to identify any illegal activity through IT checks, emails, intranet and internet usage and checks from Mba's office telephone.

Investigators also circulated Leigh's office and mobile number among staff so that they could be cross-referenced with Mba's numbers.

A memo sent in December 2011 said the revenue had checked Leigh's details but found no evidence of contact with Mba.

Leigh, who retired from the Guardian last month, said: "The revenue's decision to use these powers to try and find a link with a journalist when the disclosure was so obviously in the public interest was heavy-handed and foolish, and shows the level of paranoia over their tax deals."

Mba was suspended from work, as the Guardian revealed on 8 December 2011, when public accounts committee members warned revenue officials not to harass or bully him. However, the organisation continued to receive and detail his phone records, documents show. The inquiry was abandoned on 11 January 2012.

Mba, who trained as a barrister in Nigeria and completed his master's degree at Oxford, worked in the personal tax litigation team that dealt with the Goldman Sachs tax issue. He told the NAO and two parliamentary committees that the bank's settlement had been agreed with a handshake by Hartnett, the permanent secretary for tax at HMRC.

Mba believed the deal could be illegal, and told auditors he was making the disclosure under whistleblowing legislation. His evidence led to Hartnett's being accused of lying to parliament over his role in the Goldman Sachs deal, which he denied. He admitted, however, that his organisation had made a mistake by approving the deal.

In June 2012, Mba filed a claim under the Public Interest Disclosure Act in the central London employment tribunal. In November 2012, HMRC ordered Mba to return to work in a different team.

In 2011, HMRC was authorised under Ripa to view 14,381 items of "communications data" on taxpayers while investigating tax evasion, compared with 11,513 items in 2010, according to figures released under the Freedom of Information Act.

The employment tribunal claim continues and is expected to be heard in the autumn. HMRC declined to comment when contacted on Monday.