Swansea and Cardiff are among the most overcrowded in Britain

Its survey found Swansea jail is the fifth most overcrowded prison in the two countries, while Cardiff jail comes in at number six.

And Cardiff, Swansea and Parc Prison, in Bridgend, are also above the England and Wales average for the number of positive drug tests carried out.

The overcrowded conditions mean Welsh prisons also failed to meet the recommended average of 24 hours a week of purposeful activity for each prisoner.

The Prison Reform Trust said the situation in Welsh prisons is still better than the majority of those in England.

But the Trust's Juliet Lyon told BBC Wales that the overcrowding did, nevertheless, cause operational problems.

"Cardiff and Swansea have around three-quarters more prisoners than they should have," she said.

"People are going to be locked up for a lot longer," she added. "It's going to be difficult to organise basic things like purposeful activity, showers, and contact with family - all the things that make a difference."

The Trust believes the problems will not be solved until the courts make greater use of community penalties and stop seeing a prison sentence as the only option after conviction.

Report author Enver Solomon said: "This reveals a prison service severely hampered by a record prison population.

Cardiff's city centre prison has cramped conditions

"Far too many prisons are overcrowded, provide inadequate levels of purposeful activity and experience high levels of recorded drug use.

"The only way to improve these jails is immediately to reduce their populations."

The Prison Reform Trust report revealed that 2002 also saw a record number of suicides in England and Wales - 105 - and more prisoners than ever also tested positive for drugs.

Prisons in England and Wales failed to meet their overcrowding target, with 20.4% of the average population doubling up in single cells, against a target of 18%.

Shrewsbury was the most overcrowded jail in England and Wales with a 91.1% average.