Cardiff and Swansea are among the top 10 most overcrowded prisons in England and Wales, according to new figures released on Friday.
Cardiff prison is the most overcrowded in Wales
The Prison Reform Trust study found 79.4% of Cardiff inmates are doubling up in cells designed for one, making it the fifth most crowded jail.
Swansea came in at eighth, with 71.6% of prisoners sharing.
A Prison Service spokesman said jails were making progress, considering the overall prison population.
The figures also showed Cardiff had the seventh highest incidence of drug use, with 25.3% failing random tests.
The Victorian-era building has moved up one place from sixth in last year's Prison Reform Trust (PRT) report, while Swansea, at number five last year, had fallen three places.
Inspectors in 2003 found Cardiff was holding 150 more prisoners than it should be.
They also concluded drug addicts held there were not being given enough help to kick their habits.
The 2004 report compared the Prison Service performance targets published last months with actual results.
Across Wales and England, the study found the service had failed to meet seven of its key performance indication targets.
These included a target of 10% for positive random drug tests (actually 12.3%), and a serious assault rate of 1.25% (actually 1.54%).
Young offenders institutions scored much higher in assault statistics at 4.32%.
There was some good news for Welsh jails - Usk in Monmouthshire was among a group of 11 establishments rated as being the safest, with no recorded assaults.
PRT spokesman Enver Solomon said jails were struggling to provide useful activities for inmates, such as work or education, as prison populations remained at record levels.
He said the situation in Cardiff and Swansea was "very bad".
"Those two jails are facing what can only be described as chronic overcrowding," he said.
"Things have deteriorated over the last financial year.
"It means prisoners don't get out of their cells as much as they should - they don't get into activities that are going to rehabilitate them."
The report found there had been a significant increase in the amount of drugs used by prisoners, along with a rise in violence levels.
In some prisons, inmates have to go to the toilet in front of each other, and even eat meals seated on a toilet because of lack of space, the report found.
A spokesman for the Prison Service said of the PRT findings: "As we said when we first published the
figures, the Prison Service is pleased with what we have achieved.
"I don't think the PRT has recognised the improvements we have made in the
"In the light of the fact that the prison population has been quite high, we believe progress has been good."
Swansea Governor Phil Taylor said that having two people in a cell can work in the prison's favour.
"If we've got a particularly vulnerable individual, we can place them with a more mature individual who can mentor them, he said.