A HM Inspectorate of Prisons report, following a visit in January, also found the use of restraint by staff on children had "unquestionably" increased as well as the number of young people self-harming.
Despite improvements recorded on a visit last year, inspectors said the institution had been left without a governor for five months which had contributed to a decline in safety and care.
"A new governor was now in post and beginning to stabilise the environment, but it was evident to us that there had been a degree of drift resulting in deteriorating outcomes, notably in safety and care," the report said.
A survey carried out by inspectors found 13 per cent of children felt unsafe and that not all violent incidents were properly investigated.
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The report said systems in place to manage the perpetrators of violence and to support victims were inadequate.
"The prison relied on a complex matrix of keep-apart protocols to keep violence at bay but had not developed an effective strategy to tackle the root causes of violence," it said.
Furthermore, the investigation found 64 per cent of children said they had been physically restrained with around 400 "use of force" reports yet to be completed.
Roll checks for the institution - which at the time of the visit had 140 children of a maximum capacity of 180 - revealed 26 per cent of children were locked in their cells during the working day.
"A situation that was worse than last year and overall very poor," said the report, adding: "However, there was evidence of real improvements to the education and training curriculum and to the management of teachers."
Inspectors highlighted a number of recommendations for improvement arising from the 2018 inspection, that had not been acted upon.
"Last year we reported on a much-improved institution where good leadership had resulted in outcomes across three of our healthy prison tests - safety, care and resettlement - being reasonably good," they said.
However, of 51 recommendations made overall last year the prison was found to have achieved only 18 of them, partially achieved four and not achieved 29 of the recommendations.
Responding to the findings of the report the Howard League for Penal Reform said the inspection had revealed "an unsafe and deteriorating prison with alarming increases in violence and self-injury".
The charity said its confidential legal advice line for children and young people in custody had received 57 calls in the last year, either from or on behalf of children in Feltham, the highest call rate about a children's prison.
The most common concerns raised were in relation to the treatment and conditions experienced by children, as well as preparing children for resettlement on release from custody, it said.
One call concerned a teenager facing disciplinary measures for criminal damage despite the incident involving the punching of an object in order to self-harm.
Frances Crook, the charity's chief executive, said it was a "scandal" that thousands of children had experienced poor treatment and violence.
"It is absolutely depressing, but not surprising to see Feltham once again slump into further problems around safety, after slight improvements registered by inspectors last year.
"With such high levels of violence, escalating self-injury among children, and nearly two-thirds of boys experiencing physical restraint, the truth is Feltham is an irredeemably flawed institution," she said.
Dr Jo Farrar, chief executive of HM Prison and Probation Service, said urgent action was being taken to address the concerns raised, which included the opening of a specialist unit to provide interventions and support for the most challenging young people.
Each young person would also be provided with a dedicated officer to help their rehabilitation, she added.
Farrar said an extra 90 prison officers had been recruited since the last inspection and more than 50 youth justice specialist officers were being trained.
"We know that there is a lot more to do and that significant change is needed which is why the governor and her staff will continue to work hard ahead of the return of the inspectors in July," she said.