Latest data ranks prisons in Cyprus, Italy, France and Sweden among the most overfull.
The number of prisoners in the EU increased in 2021, with eight member states experiencing overcrowded cells, according to the latest data published by Eurostat.
The continent-wide figures rose to 475,000 – a 2.5% increase compared to the previous year - equating to 106 prisoners per hundred thousand EU inhabitants in 2021.
Prior to that, the number of prisoners in the EU grew significantly by 24% from 1993 to 2012, whilst the population only grew by less than 5%.
Some 14 out of 26 countries that were included in the statistics observed an increase in the rate of prisoners after a COVID-influenced dip in 2020.
While Hungary and Poland accounted for the joint-highest prisoner rates, Finland and Slovenia recorded the lowest prisoners-to-inhabitants ratio.
Despite being the second lowest since the turn of the century, 2021 saw prisons in eight EU member countries plagued by overcrowding.
The figures come after WHO's recent report found more than a third of all prisoners in European jails were suffering from mental health disorders due to "issues of overcrowding."
Cypriot prisons were the most overcrowded in the EU, with almost 50% more prisoners than the official capacity.
The US State Department's Human Rights report denounced Cyprus for its overcrowded prisons in 2022 with many asylum seekers forced to tolerate harsh conditions.
Malta's figures showed the country still had one-third of its prison space vacant. In total, 17 countries had "extra cells" despite increasing prisoner rates.
Official 2021 figures were not available for Belgium, but in a sign that the situation is poor, the country recently released 300 inmates in order to avoid overcrowding.