A Liberal Democrat justice minister has broken ranks and criticised government prison policy as kneejerk and unsuccessful.

Simon Hughes, minister of state for justice and civil liberties, told the Independent he believed the prison system was in crisis, something the Conservative justice secretary, Chris Grayling, has persistently denied.

Hughes said the Tory-led coalition’s response amounted to a “sticking plaster”, describing prisons as a revolving door with many criminals re-offending shortly after being released.

The latest Ministry of Justice statistics show that the re-offending rate for adults released from prison between January and December 2012 was 45.2%, while for those who had served less than 12 months in custody the figure rose to 57.6%.

Meanwhile the prison population stands at around 85,000 men and women, assaults against prison staff are at an eight-year high and suicides have risen 38%, according to MoJ figures released last week.

Hughes, who has responsibility for women’s prisons, told the paper: “The Tory approach is a traditional Tory kneejerk one, which means you have to be tough on crime and have to sound tough on crime. We have always argued that you should look not for what sounds tough but what is effective.

“The crisis in the prisons is not a lack of capacity. It is that there are too many people in there who ought not to be in there – sometimes because they are mentally ill. That is one of the causes of the assaults and the pressures.

“My analysis is that the solution is not a sticking plaster – a few extra people recruited here, or a better regime there. It is reducing significantly the number of people in our prisons so they have time for the education, training, all the productive things that make rehabilitation more likely to succeed.”

His comments come just days after another Liberal Democrat, Norman Baker, resigned from his Home Office post over disagreements with the home secretary, Theresa May.

In September, Grayling denied any prisons crisis, telling the Commons: “The truth is we have space in our prisons, they are less overcrowded, we are increasing education, they are less violent than they were under the last government.”