A Manchester prison with “catastrophic levels” of drugs, organised crime, high rates of violence and a rat infestation is in urgent need of improvement, an inspection has found.

HMP Manchester, previously known as Strangeways, was found by HM Inspectorate of Prisons to be the most violent of all adult men’s prisons in England and Wales, with the highest rate of serious assaults.

HM Chief Inspector of Prisons Charlie Taylor said: “This was a very difficult inspection with my team shocked by the level of decline they found in Manchester.

“The prison was fundamentally not safe enough for those living and working in there. Drugs present a critical threat to the security of our prisons and it was very disappointing to see that some key measures in the physical security of a prison like Manchester, holding many men with known links to organised crime, were inadequate.

“The prison service must act urgently to address this and to support staff to manage a very challenging population of prisoners. But Manchester is not alone in facing these challenges which are becoming endemic in many parts of the prison estate and demand the attention of the Government.”

In an urgent notification letter sent to Justice Secretary Shabana Mahmood, Mr Taylor said the number of weapons and illicit items found in the prison in recent months was among the highest of all adult men’s prisons and 39% of prisoners had tested positive for drug use in the last 12 months.

More than half of inmates said they had felt unsafe at some point and there had been six self-inflicted deaths and three deaths with suspected links to drug abuse since the last inspection, the letter said.

Inspectors found the prison was “filthy”, with a chronic rodent infestation, and many cell windows were smashed, prompting prisoners to use foam from mattresses and pillows to push into window frames to stop the cold getting in.

Prisons minister Lord Timpson said an action plan to deliver urgent improvements would be published in the coming weeks.

He said: “Reports such as this have become far too common. They typify the shocking state of the prison system we have inherited.

“Every day our dedicated staff do vital work keeping the public safe. They deserve better than being faced with conditions such as these and the constant threat of violence.

“That is why this Government took immediate action to end the overcrowding crisis engulfing our jails. We will now continue the hard work to ensure prisons like HMP Manchester become places that create better citizens and not better criminals.

“The Governor and his team continue to do a great job in very difficult circumstances and are already working to address the inspectors’ concerns.”

HMP Manchester, which houses category A and B prisoners, was the scene of the longest prison riot in British history, lasting from April 1 to April 25 1990.

Andrew Neilson, director of campaigns at the Howard League for Penal Reform, said: “Drugs are a scourge in prison, leading to debt and ultimately violence and self-harm.

“Security at Manchester prison must be tightened, clearly, but ultimately the way to combat drugs is to reduce demand for them. This means ensuring well-trained staff get men out of their cells and into activities that will help them to live healthily and move on from crime.

“This is the scale of the challenge confronting the new Government. Instead of wasting resources on building new jails, we should focus on restoring order and decency to the ones we already have.”