Some sentencing has been criticised as 'ill-considered' and 'badly-drafted'
Prison sentencing should be overseen by a new independent body to ensure the courts are not politically influenced, the Prison Reform Trust has said.
The pressure group said a sentencing commission in England and Wales would advise judges, prevent "sentence inflation," and tackle overcrowding.
The trust said the body could also give information to the public, who were "very badly informed" on the issue.
The number of prisoners in England and Wales reached a record 83,171 in June.
Professor Mike Hough, one of the authors of the trust's report, said something needed to be done to "stabilise sentencing".
People are very badly informed about sentencing and they need a source of honest, accurate information
Professor Mike Hough
Prison Reform Trust
He told BBC Radio 5 Live the trend that had seen the prison population almost double in the past 15 years, needed to be be stopped.
Professor Hough said: "There already is a Sentencing Guidelines Council, which gives advice to judges, but we think there's scope for a rather more substantial sentencing commission that could advise judges.
"It could stop the upward drift in sentencing. But more importantly, it could probably give information and details about sentencing to the general public.
"People are very badly informed about sentencing and they need a source of honest, accurate information."
The report also criticised the government for introducing "ill-considered, badly-drafted legislation," which it says has exacerbated overcrowding.
The trust singles out in particular so-called "indeterminate sentences," in which inmates serve a minimum term, but then have to prove they pose no risk to the public before they can be released.
The report says more than 4,000 people are serving these types of sentences, with 600 already beyond the minimum jail term.
The recommendations for a sentencing commission come shortly after the government announced proposals for an American-style sentencing framework intended to make sentencing more transparent and prisoner numbers easier to predict.
But the trust said these plans would be unpopular with judges as it would limit their discretion.