The group has lobbied against the death penalty and praises the government’s promise to commute death sentences to life imprisonment.
But that makes improving prison conditions even more important, the report says.
Thai prisons have an official capacity of about 100,000 prisoners but currently hold more than twice that number, according to the government’s own figures.
As a result, the conditions are horrendous, says the group.
People have to sleep in tight rows on hard floors, each prisoner having an average of one square metre of floor space in the “dormitories”.
Danthong Breen, chairman of the UCL, says the overcrowding is shocking.
“In the women’s prisons it’s particularly bad,” he said. “You [can] have 200 women in a single cell. If one of them has to get up at night to go to the toilet, they all shift a bit and when she comes back the space is gone and she has to stand up for the rest of the night.”
The widespread use of shackles is the other horror.
These are welded on to the ankles of long-term prisoners and are not removed, even during illness, until the sentence is served.
The United Nations has criticised the practice and the Foreign Ministry has promised to work to end it. The Constitutional Court has ruled that the practice is wrong.
But the reality of over-crowding and prisoner-to-warder ratio of about 45:1 makes it hard to stamp it out.