The claim: England and Wales have the highest incarceration rate per head of the population in Western Europe.
Reality Check verdict: In 2014 England and Wales did indeed have the highest incarceration rate in Western Europe, marginally ahead of Scotland.
Together with former home secretaries Jacqui Smith and Ken Clarke, he had written a letter to the Times calling for the prison population in England and Wales to be cut from the current level of more than 85,000 to the level in 1993 of about 45,000.
They referred to 1993 as being the year in which Michael Howard said that "prison works".
The prison population grew rapidly after that point, especially in the following five years. In the last five years it has been pretty much static at about 85,000.
Since 1993 the prison population has almost doubled over a period when the overall population of England and Wales has grown by about 17%.
Mr Clegg was only talking about England and Wales because in the rest of the UK the justice system is a devolved power, controlled by the Scottish Parliament and the Northern Ireland Assembly.
He also said: "We already have higher rates of incarceration per head of the population than anywhere else in Western Europe."
This chart based on Eurostat figures for 2014 shows that England and Wales is indeed ahead of the rest of Western Europe with 146 prisoners per 100,000 population, narrowly ahead of Scotland's 145. Northern Ireland had 93 prisoners per 100,000 population.
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Former deputy prime minister Nick Clegg was on Radio 4's Today programme on Thursday talking about prison reform.