A solicitor at HM Revenue & Customs who turned whistleblower to disclose that senior managers had quietly let off Goldman Sachs from paying millions of pounds in tax penalties is facing disciplinary procedures and possible prosecution for speaking out.

Osita Mba has worked within the Revenue for at least four years and claimed to have personal knowledge of the deal that allowed the bank to write off a £10m bill.

He told the National Audit Office and two parliamentary committees the bank's settlement had been agreed with a handshake by Dave Hartnett, the permanent secretary for tax at HMRC.

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

Now Revenue & Customs managers have launched an inquiry into Mba's conduct that could lead to his being sacked or face prosecution for disclosing sensitive information.

He has been told not to enter the Revenue's building in Westminster without a personal escort from his line manager and has been summoned to a meeting, according to documents sent to the Commons public accounts committee.

Mba's treatment has angered MPs who say that his disclosures have been of enormous service to Parliament and the public. Margaret Hodge, Labour chair of the committee – which uncovered the deal using Mba's evidence – said: "Whistleblowers play such an important role that the previous government brought in legislation to protect them.

"Mr Mba's evidence has been crucial in uncovering not just specific but systemic problems in HMRC's secretive relationship with big corporations.

"This is harassment and is completely unacceptable."

Stephen Barclay, a Conservative MP on the committee, said it would ask further questions about HMRC's approach to whistleblowers. "We have already seen how whistleblowers from the health world have been stopped from disclosing information by their managers. It appears that a similar obstructive approach may now be being followed by Her Majesty's Revenue & Customs.

"The committee will be making further inquiries to establish the facts to ensure that the will of parliament is being followed," he said.

Hartnett reached an agreement with Goldman a year ago whereby it did not have to pay back about £10m interest on tax it had avoided.

The bank was found to have employed London-based workers in the British Virgin Islands where they did not have to pay tax on bonuses.

Mba, who trained as a barrister in Nigeria and completed his master's degree at Oxford, worked in the personal tax litigation team from February 2007 until November last year.

He wrote to Amyas Morse, the auditor general of the National Audit Office, in March, outlining his concerns over the deal. The subsequent NAO report did not name Goldman Sachs.

Hartnett discussed the NAO report with the Treasury select committee in September and said the Goldman settlement was properly reached.

Hartnett denied any personal involvement in the settlement.When asked by Jesse Norman, a Conservative member of the committee, whether he had ever received corporate hospitality from Goldman Sachs, Hartnett responded: "I have been to a supper with Goldman Sachs … I knew nothing of Goldman's tax affairs when I was at that supper. I do not deal with Goldman's tax affairs."

In October, Mba sent a detailed submission to Hodge and Andrew Tyrie, the chairman of the Treasury committee, claiming that Hartnett had misled the committee over his role in the Goldman Sachs deal.

The public accounts committee accused Hartnett and senior members of staff of misleading them, and hiding behind "client confidentiality" to avoid telling the truth about the Goldman tax deal.

Mba's identity was revealed on Thursday in evidence released by the committee.

A senior judge is to be brought in by the National Audit Office to investigate such highly controversial tax deals. The NAO is considering whether to examine the tax affairs of other big companies such as Vodafone to establish whether HMRC officials routinely signed off deals that underestimated the true liabilities of the companies.

An HMRC spokesman said it had begun an inquiry into Mba and confirmed that this could lead to his dismissal and prosecution.

Insiders say that section 18 of the Commissioners for Revenue and Customs Act 2004 states that information held by the organisation must be held purely for the business of the organisation.

"It is essential that we establish the full facts in order to decide what action to take," the HMRC spokesman said.

"To this end we are carrying out a preliminary leak inquiry following the disclosure, including on the Guardian website, of internal departmental documents which may constitute a serious breach of confidential information.

"This is the first time we have had an opportunity to see the documents and we will give careful consideration to this material."