A call for "honesty in sentencing" at the courts has been made by MP David Davies who wants to see an end to all forms of early prison release.
The Tory MP for Monmouthshire also wants to stop the tagging and licensing of criminals.
It comes in the wake of a sentencing debate following the jailing of paedophile Craig Sweeney in Cardiff.
But the Prison Reform Trust says prisoners released early on tag have lower repeat conviction rates.
Mr Davies, who launched the Stop All Forms of Early Release (Safer) campaign in London on Wednesday, said: "It's all about honesty in sentencing.
"A lot of people will be released before the half-way point of their prison sentence and allowed out on a tag.
"It's not good that the public will have read that someone has been given a four-year prison sentence and will not find out that that person will be allowed out after one year and seven-and-a-half months with a tag. That is quite normal these days.
"Most people think you'll get a bit of time off for good behaviour - most people don't realise that you are automatically released half-way through your sentence and very often prior to that with a tag.
"I think there is a good argument for more prisons and more people need to go to prison, and have better facilities in prison, but initially what this is about is honesty in sentencing."
The MP highlighted the recent case of Craig Sweeney who abducted and sexually assaulting a toddler from her home in Cardiff.
Sweeney admitted four charges relating to the kidnap was sentenced to life but can seek parole after five years.
But Juliet Lyon, from the Prison Reform Trust, said while she agreed on some points Mr Davies made, she stressed that the current system does reduce the rate of reconviction of offenders.
"I think there is some confusion about what happens in court, for example, when a judge hands down a life sentence it very rarely means whole life," she said.
"There might well be a case for calling different sentences by different names.
"The other thing that I think is confusing and not always explained properly is that if someone is on a life sentence of whatever length in prison, they will spend the rest of their life under supervision in the community so the sentence continues but the rest of it is served in the community.
"Where I don't agree with David Davies is that there is a need to get even harsher, given that the courts have got an awful lot tougher in the last 10 years and there is clearly documented evidence about how much tougher they are.
"What we are seeing is with people who are getting tagged, the rate of reconviction is very low compared to people who simply leave prison in the ordinary way - we are talking about over 90% not reoffending while they are on a tag.
"Whereas I'm afraid we are talking about an average of 67% of people reoffending - in other words two out of three - in the ordinary way of things which is a pretty poor rate. The reconviction rate is shocking."