Derbyshire's police force has been fined £60,000 after officers were burned by petrol bombs thrown at them during riot training.
Four out of the 13 taking part suffered lower body injuries during the exercise in Rotherham, South Yorkshire, on 2 February 2021.
Three needed hospital treatment but all returned to work.
The officers were wearing flame-retardant gear but the Health and Safety Executive (HSE) said they weren't given enough information on how to properly use it and were exposed to "significant and avoidable risks".
It said they suffered permanent scarring and lasting psychological harm.
The risk assessment was also said to be insufficient for the "production and deployment of petrol bombs", and the force "failed to implement safe systems of work to control the foreseeable risks".
Derbyshire Constabulary pleaded guilty to breaking health and safety laws at a hearing on Monday at Sheffield Magistrates' Court, where it was also told to pay £9,470 costs.
HSE inspector Jennifer Elsegood said it was important to train police for dangerous situations but "it must never come at the expense of their safety".
"High-risk training activities must be planned and controlled with the same care and professionalism expected in any other workplace," she said.
"The risks created by the training should have been identified as part of the constabulary's risk assessments and appropriately controlled."
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Derbyshire's Chief Constable Rachel Swann said the force had taken the matter "very seriously" and had "learned the lessons from this failure".
She said the risk assessments found to be inadequate related to the handling, transport and delivery of petrol and weren't linked to the injuries suffered by the officers.
"We recognise that the risk assessments should have been regularly reviewed and updated and acknowledge that in failing to do so officers were put at risk, and we have apologised to those involved," added the chief constable.
"We always look to prioritise the welfare of our staff and the public and this court finding reflects our acknowledgement that on this occasion we failed to do so."

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