A prison governor who reached two unlawful verdicts on a documentary watched by 1.4million viewers is being forced to undergo training.

The official told Durham prison inmate Tommy Calder that staff were 'allowed to strike' convicts if they felt threatened - a breach of rules on the use of force - after the convicted appeared with a bloody face before Channel 4 cameras.

Unlawful adjudications aired in the final episode of three-part series Prison and the verdicts have now been quashed with the decisions placed under review.

The governor at Durham prison (pictured) was found to have reached two unlawful verdicts in a case that was shown in the final episode of Channel 4's Prison documentary

Calder (pictured) was left bloodied and alleged that he had been assaulted, but the governor ruled against him in a decision that has now been quashed

The head of the Prison and Probation Service has written to a charity for inmates admitting the failings, the Guardian reports.

The final episode featured details of an incident in which staff removed a television from an inmate's cell and placed him on a basic regime.

Tommy Calder, the convict in question, accused staff of attacking him as they removed the set. One officer said that Calder kicked him and that he punched him in retaliation.

Although the alleged violence was not filmed, footage of the incident's aftermath showed nine prison officers escorting Calder from his cell as the inmate wore only underwear and had a badly marked face and one eye closed.

An investigation into the alleged assault by a member of staff ensued. It also probed whether Calder had been in possession of a weapon and had attacked the officer. The prison governor found that Calder was guilty on both counts.

She said that the inmate had given her 'no reason to doubt the word of the officer'. The governor said she found the assault proven 'on the balance of probabilities'.

The governor was ruling on allegations of assault at this prison and ruled on the basis of the balance of probabilities

Balance of probabilities is the proof required in civil cases, compared with the 'beyond reasonable doubt' standard in criminal cases.

Prison standards state that charges against inmates must be dismissed if they are not proven beyond reasonable doubt.

When Calder asked the Durham prison governor if she had seen pictures of his injuries, she said that the use of force 'is not always clean and tidy' but officers can strike if they feel threatened.

The prison service instruction 47/2011 rules on the use of force in jails is 42 pages. It states that force is lawful if it is necessary and proportionate to the seriousness of the situation.

Regulations warns that blows to the head might fracture the skull of lead to a brain haemorrhage.

The handbook does not say that prison staff are permitted to strike inmates when they feel threatened.

Calder was sentenced to a loss of privileges for 28 days and the investigation continues into his claim that the staff previously exonerated assaulted him.

The Prisoners' Advice Service (PAS) director wrote to the chief executive of the Prison and Probation Service Michael Spurr to highlight the unlawful nature of the governor's findings.

The letter asked Spurr to inquire urgently into whether other governors in Durham had misapplied the standard of proof required in adjudication.

Spurr replied that the ruling was unlawful and said that the the governor would undertake training and faced reviews of every adjudication over which she has presided.

Prisoners' Advice Service Lubia Begum-Rob said she is pleased the prison service has acknowledged its responsibilities. But she also worries that the finding could be the tip of the iceberg.

She pointed out that thousands of disciplinary hearings occur each week in British prisons.

'There is no provision for legal representatives to attend disciplinary hearings in front of prison governors,' she added. 'Legal aid for advising on them was cut in 2013, and the proceedings are not tape-recorded.'

She argues that because of this there is no way of knowing how widespread problems are.

A spokesperson for the Prison and Probation Service said it is reviewing the governor's adjudications and has reminded adjudicating staff how they are to perform their duties.