An Oxfam trustee has resigned from the charity’s board over claims of governance failures and “cruel and inhumane” treatment of the organisation’s former boss.

Dr Balwant Singh said he had “lost confidence in the board’s governance, integrity, transparency and accountability” a month after Halima Begum was forced out as chief executive. “These failures are now sufficiently serious and systemic to warrant external regulatory intervention,” Singh said.

Begum left the organisation in December amid concerns about her leadership – with an apparent split emerging among members of the charity’s board of trustees.

In a resignation statement published by the Observer, Singh, a trustee of Oxfam since November 2022, said: “I feel morally obliged to resign from the Oxfam GB board and my last act before I leave is to apologise to [Begum] for the cruel and inhumane way she has been treated. We should have been better than this.

“These governance failings indicated the systemic problems faced by Oxfam GB. The Charity Commission must now remove [this board] and launch an independent investigation into governance failures.”

Oxfam announced on 9 January that it would commission an independent review of its board processes after Begum’s departure. It came after media revelations of infighting among the senior leadership at Oxfam, one of the country’s biggest charities.

Oxfam’s chair, Charles Gurassa, who is also chair of Guardian Media Group, stood down in November. Begum had filed a grievance claim against him before his departure, a complaint that he told the board was “inaccurate, with mischaracterisations and innuendo”, the Observer reported.

In its announcement, Oxfam said the employment investigation firm Howlett Brown had been appointed to examine continuing concerns about the leadership of Begum, who was appointed as chief executive in late 2023. After the leadership review, the board made the unanimous decision in December that trust and confidence in her had been lost, the charity said.

Oxfam said the independent review would assess whether the board and trustees acted in line with their duties and policies after the departure of Begum and, if relevant, before that.

After Begum’s departure was first reported in the Times, Singh said the statement made on behalf of the board had not been shared with him and did not “reflect my views as a trustee”.

He condemned the “brutal” briefing against Begum and said the findings of the leadership investigation were not shared with her and she was not given a right of reply.

A lawyer representing Begum said last month that she had been subjected to “a victimising witch-hunt”.

After a request for comment, Oxfam referred the Guardian to its statement on 9 January. It said: “As this review is independent, Oxfam is unable to comment further at this stage, to ensure the scope, process, and outcome are not affected.”