An investigation is under way after government officials altered and redacted documents in an employment tribunal case involving a bisexual prison officer, the Guardian has learned.

The judge, Michael Ord, sitting with two lay members in an employment tribunal in Cambridge, found earlier this year that Ben Plaistow, 41, experienced a campaign of direct discrimination and harassment because of his sexuality, and was victimised and unfairly dismissed because he complained about what took place.

He was subjected to a litany of abuse including being called“poof”, “gay” and “vermin”, was pushed and slapped, and had a bottle of water squirted in his face.

Plaistow was screamed at, a colleague dug her fingernails into his face, the logo on his work bag was coloured pink and a pink fairy cake was smeared inside his work bag. One colleague threatened to put him “on his arse” and another told him he was “causing too many problems” because he complained about the ill-treatment. He was asked to disclose his sexuality during his induction at HMP Woodhill.

When he lodged complaints, which were investigated internally, he received no redress. Two of the things his colleagues found fault with were the fact he dyed his hair and wore well-pressed shirts.

The tribunal found the Ministry of Justice had failed to disclose documents, carried out “inappropriate redactions” and altered documents.

“The credibility of the respondent’s [Ministry of Justice] evidence is sadly lacking. Two documents have been altered without explanation,” the tribunal judgment states. The MoJ has confirmed an internal review into the case is under way.

Seventeen officials gave evidence against Plaistow, including six prison governors, six prison officers and several other officials, some in senior roles. One prison officer who was involved in many of the allegations did not give evidence, although no explanation for this was given.

The tribunal praised the evidence of one of the witnesses, the governor, Olivia Kerr, but criticised the evidence of some of the others.

It found all the senior officers, with the exception of Kerr, “were guilty of obfuscation and gave evasive answers to often the most simple of questions”.

In contrast, Plaistow was found to be “a straightforward and honest witness”.

Some of the officials who gave evidence against Plaistow have been promoted since the incidents of harassment and discrimination occurred; two have been awarded OBEs in the past year.

The prison has come under fire for its failure to prevent suicides and most recently for the death of Daniel Dunkley.

Plaistow was hired as a prison officer at HMP Woodhill in 2014 and was fired two years later after he was accused of gross misconduct. In December 2015, he was alleged to have used unnecessary force on a prisoner while breaking up a fight. The incident led to his suspension and subsequent dismissal.

But the tribunal heard he did not use unnecessary force. This was backed up by CCTV footage and witness testimony from the prisoner, who acknowledged he was at fault.

The tribunal found Plaistow had been dismissed because of his complaints, rather than any alleged misconduct. It was highly critical of the decision by government legal department officials to redact certain documents.

“The failure to give proper disclosure in this matter is in our view inexcusable,” the judgment states, and adds that it was “designed to hide the truth from the tribunal”.

Plaistow wrote to his MP, Andrea Leadsom, about the abuse he was experiencing. But the MoJ did not disclose this letter until very late in the proceedings.

The tribunal concluded: “There is a pattern of conduct within the senior management of HMP Woodhill. It is a pattern of conduct which turns a blind eye to the bullying conduct which the claimant suffered.”

An MoJ spokesman said: “We do not tolerate any kind of discrimination in our prisons and take action to make sure all staff are treated appropriately and fairly.

“A new staff network – Pride in Prison and Probation – was set up in 2016 to support workplace equality and now has 5,000 members. We have noted the judge’s decision and are considering next steps, and a separate internal review is ongoing.”