Rising levels of violence, self-harm and suicides behind bars threaten to "severely undermine" the Government's prisons overhaul, a parliamentary report has warned.
Safety in jails in England and Wales continues to deteriorate "significantly" despite a host of measures designed to improve the picture, according to the House of Commons Justice Committee.
Its report raised concerns about the possible impact on the Ministry of Justice's penal reform agenda, which aims to drive down reoffending rates.
First unveiled by David Cameron earlier this year, the shake-up includes the creation of new "reform" jails, the introduction of league tables to assess prisons and moves to give governors greater control.
Justice Secretary Michael Gove has also set out plans to transform the education system in prisons and to increase the number of inmates let out on temporary licence.
Tory MP Bob Neill, chairman of the committee, said: "The ministry hoped that prison safety would stabilise. In reality it has deteriorated further and continues to do so. This is a matter of great concern and improvement is urgently needed.
"We will examine the details of the Government's ambitious penal reform agenda in due course.
"But this cannot wait. It is imperative that further attention is paid to bringing prisons back under firmer control, reversing recent trends of escalating violence, self-harm and disorder.
"Without such action, the implementation of these wider reforms will be undermined."
The conclusions follow a slew of data laying bare the extent of violence and self-harm behind bars.
Statistics published by the Ministry of Justice last month showed there were 100 apparent self-inflicted deaths in the year to March - the highest level for more than a decade.
There were more than 20,000 assaults in 12 months to December, a rise of 27% year-on-year, and nearly 5,000 attacks on staff - a jump of more than a third compared with 2014.
Earlier this month staff at Wormwood Scrubs prison in west London walked out over health and safety concerns.
The decline in safety behind bars has been linked to the influx of "legal highs", officially known as new psychoactive substances, a more challenging mix of prisoners and a higher than anticipated prison population at a time when staffing numbers had been reduced, the committee's report said.
However, it pointed out that these factors were already present at the time of a previous inquiry, "so may not fully explain the further deterioration in safety".
In a speech last week Mr Gove conceded the most recent figures for deaths in custody and violence in prisons are "terrible", adding: "There's no point trying to minimise, excuse or divert attention away from the increasing problems we face."
Responding to the report, prisons minister Andrew Selous said it "demonstrates the very serious challenges facing the Prison Service and shows how badly prison reform is needed".
He added: "We must do better at reducing violence and self-inflicted deaths, and preventing drugs entering prison. We must do more to help prisoners with mental health problems. We have to ensure prisoners can be rehabilitated so they are no longer a danger to others.
"We have secured £1.3 billion to modernise the prison estate and we have responded to staffing pressures with a national net increase of 530 officers since January last year.
"These reforms will ensure prisons are places of decency and improve public safety by reducing reoffending."
Shadow justice secretary Lord Falconer said:"This devastating report exposes the full extent of the Tory prison crisis.
"Staff shortages, overcrowding and a rise in violence have left many of our jails ungovernable and out of control. Self-harm and suicides are now at record levels and serious assaults on hardworking prison officers have surged.
"This is simply unacceptable. In the Queen's Speech, the Government must bring forward serious and radical proposals which address the issues raised by the select committee."