A snapshot of figures from 2016 showed 94,291 individuals were in jail or immigration detention in Scotland, England, Wales and Northern Ireland.
Of more than 40 administrations which took part in the analysis published by the Council of Europe, only Turkey had more inmates, with 192,627.
The study said there were 85,134 inmates and detainees in England and Wales at the end of June 2016.
Scotland and Northern Ireland had 7,657 and 1,500 respectively in September of that year.
The combined total was the highest for all of the EU member states and compared with 68,514 in France, 64,397 in Germany, 54,195 in Italy and 60,687 in Spain.
England and Wales had a prison population rate of 146.4 inmates per 100,000 inhabitants, which was higher than the Europe-wide average of 127.2.
England and Wales had one of the highest numbers of prisoners serving life, with 7,361, according to the study known as Space.
Lead researcher Professor Marcelo Aebi, from the University of Lausanne, said: “Among Western European countries, England and Wales have had the highest prison population rate almost systematically since I took care of Space in 2002.”
The prison population has come under sharp focus in recent years as jails were hit by surging levels of violence, self-harm and major disturbances.
The number of prisoners in England and Wales almost doubled between 1993 and 2016, and has remained around the mid-80,000 mark in recent years.
Penal reform groups have repeatedly highlighted overcrowding as a factor in declining standards behind bars.
The latest Ministry of Justice figures showed 83,921 people were held in prison or state-run immigration removal centres as of 16 March – down by 1,580 compared with 12 months earlier.
A Ministry of Justice spokeswoman said: “The prison population has remained relatively stable since 2010, and whilst we recognise the need to reduce these numbers we are clear that prisons are there to deliver the sentences of the court.
“Creating prisons that are safe, decent and secure will help offenders turn their backs on crime and lead law-abiding lives upon release.
“That is why we are investing £1.3 billion to modernise our prisons so they become places of rehabilitation and reform.”