Controversial prison sentences which have been criticised by the High Court should be "fundamentally reformed", a human rights organisation has said.
So-called indeterminate sentences for public protection (IPPs) have led to many prisoners being kept behind bars for longer than the minimum period set out by the court.
Justice Secretary Jack Straw has already announced he is reviewing the operation of IPPs, which were introduced under the Criminal Justice Act 2003.
The jail overcrowding crisis has meant inmates are unable to take courses to prove their fitness for release, leading to legal challenges.
Senior legal officer of law reform and human rights group Justice, Sally Ireland, said: "IPP is a flawed policy with flawed execution.
"The Criminal Justice Act dragged too many people into the IPP net and the Government has not put sufficient resources in place for them to be rehabilitated."
She added: "We hope that Jack Straw's review of IPPs will recognise the need for urgent reform."
At a High Court hearing, Mr Justice Collins ordered that a violent prisoner should be released - the third such ruling in recent weeks - but stayed the decision pending an appeal on all three cases.
The judge said: "Because of the failings of the Government, a fairly large number of IPP prisoners are likely to be released if the Court of Appeal finds the detention is unlawful. This is very worrying. It must be recognised that that the consequences are truly disastrous because I think it is inevitable that short-term lifers will have to be released whether or not they remain a risk to the public."
A report by the Prison Reform Trust earlier this month revealed more than 3,000 IPPs have been passed, many for relatively minor offences, in just two years.
