The risk of suicide and assault in prisons is being increased as a result of the Government's policy of building bigger jails, the Chief Inspector of Prisons has warned MPs.
And the head of the Prison Governors Association warned that a cost-driven shift towards larger institutions was changing the culture inside jails and making rehabilitation more difficult.
The construction of "super-sized" jails over the last decade in England and Wales has seen the number holding more than 1,000 men increase to more than 20. The Government plans to build a 2,000-place prison in Wrexham and is conducting a feasibility study for a second giant-sized institution in London, while there have been 13 closures of smaller prisons since 2010.
But Chief Inspector Nick Hardwick told the House of Commons Justice Committee that the trend towards bigger institutions was causing "greater risk on the ground".
"A consequence is that the level of risk has increased because new prisons are difficult to establish and big prisons are difficult to run, so new big prisons are very difficult to run," he told the cross-party committee.
The closure of small, local prisons has meant the Prison Service losing some of its most experienced officers, while new larger jails typically have "a lot of very inexperienced staff", he said.
"We are losing a lot of experienced staff so that is creating a risk. I think we can see hard evidence of greater risk on the ground. I don't think it's a coincidence that we are seeing the highest level of self-inflicted deaths in prisons for many years - already about 50% more this year than there were last year.
"The number of assaults in adult male prisons is up, the number of incidents at height is up. These incidents are terrible in themselves and I think that is a reflection of a system under real strain at the moment. I have concerns about what is happening in that sense."
HM Inspectorate of Prisons last year issued a heavily critical report on the privately-operated Oakwood prison, which opened in 2012 in Staffordshire and holds 1,600 prisoners at one-third of the average cost per place, for its inexperienced staff and high levels of violence and self-harm.
Mr Hardwick warned the committee that other new institutions, including Thameside and Isis in Thamesmead, south London, had "struggled", adding: "What I hope ministers won't do is underestimate the difficulties of establishing any new prison.
"The new prisons established recently - Oakwood, Isis and Thamesmead - have all struggled and these difficulties are exacerbated the larger the size of the estate."
The president of the Prison Governors Association, Eoin McLennan-Murray, agreed, telling the committee: "What we are seeing at the moment is large prisons and minimal resourcing, which is obviously driven by cost."
The 1991 Woolf Report on prisons made clear that the optimum size was around 400-500 inmates, and it was generally understood that "small prisons have better outcome for prisoners", said Mr McLennan-Murray.
"It is the relationships we build with prisoners that is how we control," he told the committee. "Other jurisdictions use coercion and force, we tend to use personal relationships.
"For that to happen, you need sufficient staff facing prisoners. While we are reducing the number of staff and increasing the number of prisoners, you're getting prisoners who feel that they are anonymous and no-one cares about them. That has a psychological impact and changes the culture in prisons. It will make them inherently more risky and I think that is a bad move.
"I think we will regret the day we thought big is beautiful. It certainly isn't unless you resource it correctly, and we are not doing that."
Mr Hardwick said it was "scandalous" that a large number of people in prison were suffering from mental problems which staff did not have the capacity to deal with, while Mr McLennan-Murray agreed that the Prison Service was "overwhelmed" by the scale of mental health needs it is confronted with.
"It's scandalous that the way we deal with so many of these people with these problems is to put them in a prison," Mr Hardwick told the committee. "I see it and I think it is not sensible.
"It is a mistake to think that prison officers with the level of training they have got can cope with the numbers of people they are dealing with. They don't have the resources or professional training they need to deal with a population with complex needs."
The most serious drug problem within jails was now prescribed medicines, which were often prisoners' "drug of choice" rather than a substitute for illegal narcotics, said Mr Hardwick. But governors are often unable to clamp down on it because they have little control over a healthcare commissioning process which involves a number of outside organisations providing medical services to inmates.
The Chief Inspector said that the GPs who come into prisons are often locums hired by surgeries which have themselves been subcontracted by providers commissioned by the NHS. They are vulnerable to being "bullied" by prisoners into providing drugs which can be traded within the jail, he said.
Problems were also being experienced with synthetic cannabinoids such as "spice", which mimic the effects of cannabis but are not picked up by testing regimes, he added.
Explaining the difficulties the healthcare commissioning process is causing to governors, Mr Hardwick told the committee: "In our view, the most serious drug misuse problem in prisons now is diverted prescribed medicines.
"If you ask me how do drugs get into prisoners, for a lot of them healthcare staff bring them in and hand them out to a queue of prisoners and then they become a tradeable commodity or a bulliable commodity.
"I understand from my specialist drugs inspectors that this reflects what is happening in the community. This isn't because they can't get hold of the real drug. This is the drug of choice.
"This is one of the dangers of the contracting arrangements. We found in some prisons, before NHS England took responsibility, you would have a commissioner who wasn't very engaged with what's happening in the prison, and a provider, who wasn't very engaged and perhaps not very good.
"They would then be sub-contracting the service to a GP service, which would be having it done by a locum who was actually going into the prison. These GPs were being bullied to provide tradeable drugs that they shouldn't.
"And you have the governor saying to me he was at his wits' end that he couldn't control the situation because the commissioning process was not in his hands.
"I think there is a lesson from that for the commissioning process generally, that we shouldn't disempower governors. He should be responsible for what happens in his prison.
"We think it is really important that there is no dilution of the governor's accountability for what happens in his or her prison. They need to have adequate control of the contract and commissioning process to be able to achieve that."