The GMB trade union is facing fresh turmoil over claims made by two of its female senior leadership team, as it heads towards a crucial general secretary election this year.

The infighting at a senior level comes as Gary Smith, the union’s general secretary, faces a potential battle to retain his job in May.

The GMB, which is one of Labour’s biggest donors and counts Keir Starmer and Rachel Reeves as members, has for years been trying to improve its culture after being found by an independent report in 2020 to be “institutionally sexist”.

The Guardian has learned it is now facing an employment claim for unfair dismissal from a female member of its senior leadership team sacked for alleged racism.

The senior staffer was dismissed after an investigation, in the months after she submitted a complaint of bullying and harassment.

After her dismissal, she emailed the union’s entire central executive committee (CEC), saying she believes her dismissal was a result of retribution linked to her formal complaint against the general secretary and other senior male employees.

She also claimed that there were contraventions of governance and urged the committee to take action against leaders who have created a culture of fear and control.

The union subsequently went to the high court to gain an injunction to demand the return of 700 documents she had retained.

A second female member of the senior leadership team, who is still employed but now off sick, also emailed the CEC saying she had reported bullying and harassment involving union leadership.

She is understood to have told the committee that she could no longer ignore the unacceptable behaviour of those in power and claimed that the union was rotten at the root and being damaged by the actions of those at the top.

In response, the GMB said the first staffer was sacked for making racist comments and it was nothing to do with her complaints of bullying and harassment which were not upheld. It said the second staff member’s allegations were investigated and not upheld.

Smith will face a reelection battle in May if he puts his name forward as general secretary again. He is believed to be facing at least one challenger after expressions of interest were invited in the weeks before Christmas.

The continued turbulence at GMB comes after years of trying to address allegations of misogyny, which led to the Karon Monaghan KC report. The Guardian understands that a follow-up of that report, conducted by Monaghan, found in late August this year that only 11 out of about 27 recommendations have been graded as fully implemented five years on, and that it still has more work to do to change its culture.

A leaked summary of the report, seen by the Guardian, said that while improvements have been made there are still real challenges at the GMB.

Some female staff at the GMB told the Guardian they felt the culture at the union was as bad as ever, but others said they were fed up with the union being painted as a difficult place to work for women when it gave great opportunities and had changed in the wake of the Monaghan report with measures such as new anti sexual harassment policies.

In a statement, the GMB said the first staff member was “dismissed from our employment due to a breakdown in trust and confidence, including matters arising from serious grievance relating to racism, with the complaints made against her by numerous members of staff being fully upheld”.

“Following legal action at the high court, nearly 700 files were returned to GMB by [her]. [She] had held this data in breach of contract and, despite numerous requests to return it, only did so when legal action was commenced. The GMB data returned related to numerous individuals, including multiple junior members of staff who bravely raised concerns about her conduct during employment with the union.”

It said all her complaints “have been properly dealt with, fully investigated and none upheld” and that “GMB has a zero tolerance for any form of discrimination, including racism, and we have acted swiftly in all matters where it is raised”.

A Unite representative for the first staff member said: “The response from the GMB union is one we challenge robustly. Our member was dismissed with no investigation and no hearing. An investigation and hearing, as set out in Acas guidance, is the minimum we would expect from any employer. The allegations have been denied throughout and are now the subject of an employment tribunal claim.”

On the second staff member, the GMB said: “GMB does not comment on matters relating to individual employees and has a duty to ensure staff wellbeing and confidentiality. We can confirm that whistleblowing complaints raised by a senior member of staff were investigated thoroughly over a three-month period in line with GMB’s policies and procedures.

“The investigation concluded that the allegations were not substantiated and found no evidence of wrongdoing. GMB takes all complaints seriously and remains committed to robust governance and fair process.”

In relation to the Monaghan report, the GMB said it had established working groups to complete any remaining tasks. John Smith, chair of the GMB national negotiating committee (NNC), said: “Staff representatives are pleased to be working together with GMB leadership on ensuring the completion of recommendations from the Monaghan report. We anticipate this work to be finished in the near future.”