Unions have pulled out of an independent investigation into allegations of bullying, racism and harassment at Hackney Council – accusing town hall chiefs of “consistently undermining the process”.

Unite, Unison and GMB’s decision was laid out in a 26-page letter to council CEO Tim Shields and mayor Phil Glanville today.

It is another hammer blow to already strained relations between the council and its trade unions (TUs), who have said thousands of its members could now strike.

The joint letter states: “We do not have confidence that the council has seriously taken on board the concerns of staff in the call centre nor that they have taken seriously the call by staff and the unions for an independent investigation.”

The investigation was commissioned following an explosive 18-page document sent to all Hackney councillors in October, detailing numerous allegations of serious issues at Hackney Service Centre, the call centre behind the town hall.

The allegations, first revealed by the Gazette, included:

• Workers’ complaints about discrimination being swept under the carpet, or resulting in them being punished for speaking out;

• A black female worker being called a member of the Taliban by a white, male colleague, who also said he would “start embracing his roots because he was white and whites used to enslave black people”;

• A manager disclosing, in front of another manager, that a black female agency worker had been a victim of domestic abuse, shouting at her: “Your partner is always pulling your hair out and you’re a walking car crash,” and: “There is always drama with you”;

• A senior white male manager making “well-known racist stereotypical comments to black staff”.

For all the Gazette’s coverage on the bullying row, click here.

Unions had first called for the probe six months earlier and say when it finally began, there was a “catalogue of basic errors at best, and fundamental flaws at worst”, with staff “unable to book sessions with the investigator as all slots had been allotted”. The town hall says six days were set aside for staff interviews, and another two days have been allocated.

Unions also say at the end of last month they were told a manager at the call centre had been “cautioning staff about how they contribute to the investigation” and immediately reported this to the council.

An investigation by the council decided no formal action be taken, a decision now being reviewed after complaints by the unions.

In the letter, they reiterated a claim made in October that “Hackney Council management have created a crisis affecting its workers, service users and residents that it is presently either unable or unwilling to solve.”

The letter adds: “The TUs have no faith in the exercise the council carried out in response to the TU concerns, nor does it appear the council are taking the investigation into the alleged interference with witnesses seriously.

“Regrettably, to staff working in the call centre and to the TUs, the council’s failure to launch a proper investigation has done nothing but serve to confirm the growing unrest amongst staff and lack of confidence in the councils ability to investigate themselves impartially.”

The unions are also unhappy about the council recruiting senior managers from the pool of managers being investigated at the call centre.

“The council appear unable to hold staff accountable when there are complaints or come to any meaningful conclusions despite the background to the investigation and the seriousness of the allegations,” they added. “Worst of all, staff are continuing to suffer intimidation and indignity on a daily basis.

“The council’s existing policies and procedures are clearly insufficient in protecting staff from abuse and mistreatment, whilst the extensive use of insecure agency contracts supports a culture where staff are disposable and bullying behaviour festers.

“It is deeply saddening and disturbing that faced with an opportunity to address the damaging allegations brought before it, the council has instead consistently undermined the process and are perceived by the TUs, its members and staff as having prejudged the outcome of the investigation before it has been concluded.”

A protest is set to take place outside the full council meeting tomorrow night, at which Labour member Heather Mendick will ask a question about the allegations.

Hackney Council says the probe will continue and will be published when it has been completed.

Mr Shields said: “This is extremely disappointing, given that unions have met with me and, separately, the mayor and cabinet throughout this process. “Our door has always been open and the unions have been fully involved in the investigation from the beginning, agreeing both the independent investigator and the terms of reference.

“The issues in the letter were directly raised with me within the last week. They are already being addressed, and I am planning on meeting with the unions within the next few days to further update them.

“The independent investigation will continue and we hope that the unions will engage with its findings on behalf of their members.”

Mayor Phil Glanville said he has always engaged with unions but added that it was not a political issue.

He said: “Staff grievances and complaints regarding bullying and harassment rightly need to be dealt with by management according to the processes the council has in place.

“It would be inappropriate and unfair on all staff for politicians to intervene in these processes, particularly at employment tribunal stage or when there is an agreed independent investigation in place.

“However, throughout this investigation both myself and Cllr Carole Williams [the trade unions cabinet member] have been actively involved in ensuring due process and the independence of the investigation.”

At the centre of the row is Natasha Johnson, a Unite rep who was sacked in October after an 11-month suspension.

Her dismissal came in the middle of an employment tribunal in which she has accused the council of racist, sexist and disability discrimination. The outcome of the tribunal has still not been decided.

At the hearing in October she told the judge a call centre manager had “a stereotype about black people” and once said to her while she rolled a cigarette: “I hope you are not smoking marijuana in work time.”

The council’s lawyer Catriona MacLaren told Natasha the comment was “at worst a poor joke” and said it was “not reasonable to have felt harassed” by the “benign” remark.

In January members of the Hackney South and Shoreditch Labour party voted “overwhelmingly” to support unions’ campaign to reinstate Natasha, who had her appeal dismissed.

Unite bosses say the council brought spurious counter-allegations against rep Natasha Johnson after she made an official grievance, saying she was “chasing up emails too quickly” and making managers feel harassed by accusing them of harassment.