Explosive claims that drones laden with “legal highs” are being flown to Pentonville inmates, who collapse in the grounds after taking them, are made in a report this morning.

The jail’s Independent Monitoring Board (IMB), which visits the Victorian facility in Caledonian Road several times a week, alleges crumbling windows mean prisoners can easily get hold of drugs, mobile phones and other items.

It also says the medium-security site is overcrowded, with 40 per cent too many inmates. Nearly 1,300 are crammed inside the prison walls. Four men killed themselves between April 1, 2015, and March 31 this year, up from one suicide the year before, but recorded incidents of self-harm have fallen from 450 to 392.

“Criminal gangs have successfully exploited the shoddy state of the prison’s windows by sending small, near-silent drones – which can be steered to window sills – with payloads of drugs, mobile phones, and other contraband,” said a spokesman for the IMB.

So-called legal highs, also known as “spice”, are properly termed new psychoactive substances, and many are now in fact illegal. Legislation to ban them en masse is currently working its way through Parliament.

There are no reports of prisoners dying after taking them, though this has happened at other British prisons in the past year.

Despite the Ministry of Justice agreeing to fix the windows in 2015, just 10 of 100 delapidated windows had been replaced by June this year, the report alleges. Ten of the worst could not be repaired as glazing had been ordered that was the wrong size.

In addition, the paper says contractors are slow to respond to maintenance issues, leaving a wheelchair lift out of action for six months and cells lacking in towels and soap.

Drugs dogs and security staff patrol the grounds to intercept parcels thrown over the walls – but the report describes these efforts as “like holding a hand up against the incoming tide” because of the condition of the windows.

The report praised efforts to keep the prison clean but said: “Despite improvements, including re-painting communal areas and some cells, conditions cannot be described as decent when most men share cells designed for one person, with a barely screened toilet, and many prisoners do not have a full complement of cell furniture.”

Authors struck a positive note on the topic of staffing, saying it was a “cause for optimism” that Holloway staff were set to be transferred when the latter shut.

But, they added: “The condition of this 174-year old prison is poor and nothing short of a massive injection of capital will improve the conditions for any but a handful of prisoners.”

IMB chair Camilla Poulton told the Gazette she did not believe the unsecured windows posed a threat to families living around the prison.

“I can’t think of any obvious risk,” she said, “other than the fact there are the criminal gangs trying to exploit [the windows’ lack of security] by sending contraband over the wall, and any potential antisocial behaviour associated with that.”

Asked why she believed the poor condition of the 174-year-old estate had not been addressed by the Ministry of Justice, she said: “A lack of money centrally is holding it up.

“We have identified particular issues we feel are really significant like the delay on replacing the windows. which are in a very poor state.”

She stopped short of calling for the prison to be shut down, saying no monitoring board would ever call for a prison facility to be shut, and praising its role serving the court system in inner London.

But, she added: “The prison needs to be replaced, which is what Michael Gove suggested should happen to Pentonville and other ageing Victorian prisons, or we need to live with the Victorian building and upgrade the facilities.”

An MoJ prison service spokesman said staff at Pentonville were “working hard” to tackle the problems raised by the IMB, adding police were involved in trying to stop drugs getting in.

“Safety in prisons is fundamental to the proper functioning of our justice system and a vital part of our reform plans,” he added.

“As the justice secretary has made clear, there are a number of factors including the availability of psychoactive substances in prisons which must be tackled in order to make our prisons safe and places of rehabilitation.”