A "terrible influx" of so-called legal highs has helped drive up rates of violence in Britain's prisons, a minister has said.
Surging numbers of inmates convicted for violence are also to blame, Andrew Selous claimed.
Figures for prisons in England and Wales reveal more men and women are dying, more male inmates are self-harming and assaults on prisoners and staff have shot up.
Giving evidence to the Commons Justice Committee, prisons minister Mr Selous said there had been a period of "huge change" with an "unexpected increase in the population" in recent years that "left us slightly shorter on officer numbers than we would have liked".
There has been a 30% rise in the number of prisoners jailed for violent offences over the last decade, he added.
"The mix of prisoners has got more toxic, if you like, at a time of change", he told MPs.
"On top of that, probably starting about two years ago or more, we had this terrible influx of Spice and Black Mamba, these psychoactive substance drugs which came in on prisons."
Spice and Black Mamba are synthetic drugs designed to mimic the effects of cannabis.
The Ministry of Justice says new psychoactive substances (NPSs) can cause violent, unpredictable behaviour and lead to prison assaults.
Mr Selous insisted the Government was taking "considerable" action to tackle all of the issues he cited.
A new law is being introduced to target smugglers attempting to sneak designer drugs into prisons while Mr Selous said a new testing facility would be a "game changer". He also said there had been a net increase of 540 prison officers in the year to September.
Michael Spurr, chief executive of the National Offender Management Service, said it would be "completely unfair" to suggest his organisation was "complacent" or have "no sense of urgency".
He told members: "I am not denying at all the issues. In fact, I've been public about the pressures that we've been facing.
"The issue of NPSs escalated in an unprecedented fashion. We are getting on top of it, I think, now."
The committee also questioned the pair about the issue of transgender inmates following recent controversies.
Earlier this year, make-up artist Tara Hudson was placed in a male prison after an assault conviction but is understood to have been moved to a female facility after a campaign.
Weeks later, Vicky Thompson was found dead after being held in a men's jail.
Mr Selous said: "We absolutely want to place prisoners in the most appropriate setting as far as they are concerned but we do have to have regard for the safety of all prisoners.
"I will continue to take a very close interest in this area. I don't want any further repeats of the tragic incident which we had recently."