Dilapidated windows at HMP Pentonville are worsening a drug problem at the London jail, its Independent Monitoring Board (IMB) said in a report.
It found blocked toilets and leaking sewage, and said the escapes and the killing of an inmate demonstrated "serious shortcomings".
The Ministry of Justice said more resources were being put in place.
Members of the IMB visit the jail several days a week throughout the year and said "aspects of the physical environment of Pentonville are inhumane".
It said: "Confining two men in a cell measuring 12ft by 8ft is not humane treatment.
"One has to eat his meal in the cell while the other may be sitting on a badly-screened toilet a few feet away."
It added: "The prison struggles to ensure the basics of decency largely due to the outsourced provider responsible for maintenance - Carillion."
It concluded: "The contract is working neither for Pentonville nor the taxpayer".
A Carillion spokesman said: "The age and condition of Pentonville prison presents maintenance challenges which our people work hard to overcome.
"Although the level of reactive maintenance jobs has increased we have continued to improve our performance against the agreed targets."
The Category B prison is holding 380 more men than the Prison Service deems it suitable for, the IMB said.
The report raised concerns about the risk to safety from the prison's old windows, whose replacement was recommended by the National Offender Management Service following the fatal stabbing of Jamal Mahmoud, 21, in October 2016.
It said: "Less than a month later two prisoners escaped through a cell window."
Matthew Baker, 29, and James Whitlock, 32, went on the run after sawing through a metal bar, clambering over roofs and swinging round a CCTV pole on a sheet.
"Until all the windows are replaced contraband received by drone or from throw-overs will continue to put the safety of staff and prisoners at risk," the report added.
"If cells cannot contain prisoners, then the safety of the public is also at risk."
A Ministry of Justice spokeswoman said: "Pentonville has been through a very challenging period. While the Board notes progress has been made in improving staff safety with body worn cameras for all uniformed staff, and new CCTV to tackle the flow of contraband into prisons, it is also clear there remains much more to be done.
"We are boosting the frontline by recruiting more staff at HMP Pentonville and for prisons across the estate.
"With these additional resources in places, Pentonville will be better able to address the concerns raised in the report and improve progress as a matter of urgency."
A prison where two cellmates made an escape bid has been labelled "squalid" and "inhumane".