Citizens Advice has found "considerable evidence" of bullying and harassment at a bureau in Wales after reviewing complaints made by two former volunteers.
Gillian Guy, chief executive of Citizens Advice, ordered the review, which deemed that the original investigation into the complaints about Cylch Conwy District CAB was partly flawed.
The investigation was conducted by Gwilym Evans and Paul Hinge, chairs of two other Welsh CABs.
Gillian Brocklehurst, one of the ex-volunteers who submitted complaints, claimed the investigation was a "whitewash" and should have been conducted by people not involved with Citizens Advice.
She wrote to Guy, whose review has uncovered numerous other failures by both the local CAB and the national body.
Citizens Advice said in a statement that it now intended to commission an independent external body to reinvestigate complaints about bullying, harassment and disability discrimination.
The review report, seen by Third Sector, says the CAB lacked "conclusive documented evidence" to support the decision by Phil Ward, its chair, to dismiss Brocklehurst for "disruptive behaviour".
It says the CAB had not referred to Citizens Advice policies on managing volunteers when dismissing her.
"The actions taken by the trustees could expose the board to accusations of unfair treatment," the review says.
It says the CAB board should apologise to Brocklehurst for "failing to follow due process and clarifying what steps have been taken to ensure that another volunteer will not have a similar experience in the future".
Six of 11 witness statements submitted to the investigation referred to instances of bullying and harassment. "It does seem that there is considerable evidence to indicate that bullying and harassment has taken place," the review says.
It concludes that the handling of complaints from Brocklehurst and another ex-volunteer, Liz Whitfield, had been characterised by "failings to understand, apply and follow appropriate procedures by all parties" and "poor communication has been a consistent feature throughout the whole process".
The review highlights a need for Citizens Advice processes and guidance on complaint handling to be much clearer and fuller. It recommends that the organisation develop a model dignity at work policy for volunteers and do more to promote its existing model volunteer policies.
It says the Citizens Advice whistleblowing policy is now "under review".
A statement from Citizens Advice said: "Citizens Advice intends to commission an independent external body to reinvestigate some elements of complaints made by two former volunteers at Conwy Citizens Advice Bureau, concerning allegations of bullying and harassment and disability discrimination.
"The decision follows a review of the original complaint handling that has found that parts of the investigative process were flawed."
Brocklehurst said she was "relieved and pleased" by the review’s findings.