Urgent intervention is needed at Feltham young offender institution, the chief inspector of prisons has warned, after a review uncovered rocketing levels of violence and self-harm.

Peter Clarke said the inspection of Feltham A, the west London unit for boys aged 15 to 18, had identified an “extraordinary” decline in safety and care for inmates.

As a result he has invoked the rarely-used urgent notification process, which requires the justice secretary, David Gauke, to respond publicly within 28 days with action to improve conditions.

In a letter to Gauke, published on Wednesday, among the problems Clarke highlighted were:

40% of teenagers said they had felt unsafe at some point during their stay.

The number of violent incidents had risen by 45% since January, despite the number of inmates falling.

The number of assaults against staff had risen by around 150% since January.

The level of self-harm had tripled since the previous inspection and was 14 times higher than in January 2017.

74% of teenagers reported they had been physically restrained by staff, with more than 700 incidents in the last six months.

Clarke wrote to Gauke that there had been a collapse of “any reasonable regime” which “has prevented many children from getting to education or training, delayed their access to healthcare, isolated them from meaningful human interaction and frustrated them to the point where violence and self-harm have become the means to express themselves or gain attention”.

He said the problems stemmed from over-reliance on “keep apart” policies, designed to prevent clashes, particularly between children from rival gangs.

Feltham has long been notorious for high levels of violence but an inspection last year found the situation had improved. However, an inspection in January this year found safety and care to have deteriorated once again and as a result the unusual step was taken of scheduling an additional review for July. It was the latter review that led to the urgent notification being issued.

Feltham A is on the same site as Feltham B, which holds young adults aged 18 to 21. The urgent notification was issued solely with respect to Feltham A, which holds up to 180 teenagers.

In April, after an outbreak of violence that resulted in more a dozen prison officers being treated in hospital, campaigners called for the institution to be closed.