Ministers have been urged to halt the planned rollout of pepper spray to prison officers after an analysis found it may have been used in breach of official guidance in almost two thirds of recorded incidents during a pilot scheme.

When the decision to roll out Pava, a synthetic incapacitant pepper spray, was announced in early October, the prisons minister Rory Stewart said it would only be used in “exceptional circumstances” to protect staff from threats of serious violence.

However, the Prison Reform Trust suggested guidance was routinely ignored by officers during the pilot, and that the rollout was “likely to do more harm than good and undermine the safety of prisoners and prison officers”.

Pava was deployed 50 times during the pilot scheme, which took place in four prisons – Hull, Preston, Risley and Wealstun – between January and June.

PRT’s analysis revealed that in 64% of incidents it was deployed by prison staff who may have contravened the guidance for its use. In 34% of cases officers used Pava without an appropriate justification; in 24% of cases its use was unsafe, and in 24% of cases an alternative option was available.

In several incidents the guidance, which states Pava must only be used when other restraining techniques had failed or were unviable, was breached for multiple reasons.

On one occasion, Pava was deployed against a prisoner who was self-harming and where there was no indication of a threat towards the officer.

In another, Pava was sprayed at the same prisoner three times in 10 minutes, including at point-blank range through the cell flap. The prisoner against whom it was deployed had clear, known and obvious mental health issues, the report said.

In a number of incidents there was no indication of a threat of harm, with Pava used merely to enforce an order, in direct contravention of the guidance.

In some cases, it was deployed against the wrong prisoner, and in others officers mistakenly sprayed themselves and other colleagues.

“The availability of such a potent weapon has immediately created a norm for its use which is different from what you intended, and which the safeguards in place – even in a closely monitored pilot – failed to control,” PRT’s director and a former prison governor, Peter Dawson, wrote in a letter to Stewart.

“Perhaps as a consequence, there is clear evidence from the pilot that deployment of Pava undermined the trust that prisoners had in officers and in the legitimacy of the authority those officers hold.”

The UK’s human rights watchdog had previously warned that the rollout of Pava would put inmates at risk of inhumane treatment, and on Saturday it reiterated those concerns.

“We do not understand how or why this decision has been taken,” said Rebecca Hilsenrath, the chief executive of the Equality and Human Rights Commission. “There is no sound evidence to support rolling out Pava spray in this way.

“We agree that it is of the highest importance that prison officers are able to protect themselves and others but such protections must not be at the expense of the basic rights of prisoners. Everyone has the right to live without fear of inhumane treatment.”

She said the EHRC would be writing to Stewart again to ask for a full impact assessment in line with legal obligations.

A spokesman for the Prison Service told the BBC there was no evidence to suggest Pava was used unlawfully during the pilot and it would be wrong to imply that cases of misuse were ignored.

Attacks in prisons rose to a record 31,025 last year, almost twice the 15,644 assaults recorded in the year to March 2008 and up 16% from the previous year, according to the most recent figures from the Ministry of Justice.

Experts have blamed the steep rise in violence on a decline in the number of prison officers.