The prison service is creaking under "unprecedented strain" because of fewer staff, worsening safety and fewer rehabilitation opportunities, according to the Prison Reform Trust
A new report by the trust blamed severe austerity cuts to the Ministry of Justice budget for creating a "race to the bottom" in prison conditions.
It comes as prison numbers soared by more than 500 in the two weeks between May 9 and May 23, compared to an overall increase of 1,496 in a whole year between March 2013 and March 2014.
The report, called Prison: The Facts, also found two-thirds of prisons are overcrowded, holding two inmates in cells designed for one.
And it criticised the use of super-size prisons and plans to build a new jail in Wrexham with a capacity of 2,000, claiming evidence suggests smaller prisons are safer and more effective.
Juliet Lyon, Director of the Prison Reform Trust, said: "These latest figures reveal a prison service having to cope with unprecedented strain.
"Ministers must heed the warning signs. Rising assault and suicide rates, fewer staff and less constructive activity, call into question the Government's commitment to safety and decency.
"Slashing prison budgets and introducing harsher regimes while warehousing ever greater numbers overseen by fewer staff is no way to transform rehabilitation."
The trust said the service was being "stretched to its limits" because of a combination of a 23% reduction in the number of prison officers at public prisons since 2010, high levels of staff sickness, the closure of 15 prisons and the transfer of two prisons to the private sector.
It warned that reductions in staffing levels impact on safety and the amount of time prisoners are able to spend engaged in purposeful activity.
The number of occasions the Prison Service Gold Command, the national group set up to deal with serious incidents and disturbances, has convened over the last three years has increased by 153% since 2011-12.
A total of 1,575 serious assaults took place in prisons last year, the highest number for a decade and a rise of more than 300 over the previous 12 months.
A Ministry of Justice spokesman said: "We are making prisons more effective and cheaper to run not by cutting services or reducing quality but by fundamentally changing the way we operate.
"Our current estates strategy has been described by the National Audit Office as the most coherent and comprehensive for many years.
"We have replaced old, inefficient buildings with newer ones that are cheaper to run.
"Our focus is not about how big a prison should be; it is about how effective it is at reducing reoffending.
Cuts to the Ministry of Justice budget are due to total £2.4bn by 2016.