Almost 200 prisoners, including murderers and sex offenders, have been freed early by mistake over the past five years, official statistics show.
The Tories, who obtained the figures in a series of written answers, said such errors can have "devastating" results.
The Prison Service said it had taken action to "tighten processes and focus managers' attention".
Meanwhile, the Ministry of Justice has said 4,000 new prison places have been created over the last two years.
The figures, received by the Conservatives, show 192 prisoners were let out in error between January 2005 and September this year.
The number of wrongly released inmates rose 45% in the last two years, to a rate of almost one a week.
Shadow justice secretary Dominic Grieve blamed "Labour's incompetent mismanagement of the justice system".
"Although the number of releases in error may be small as a proportion of all releases, they can have devastating consequences," he said.
"As prisons have become more overcrowded, staff overworked and transfers more frequent, the number of erroneous releases has shot up."
A Prison Service spokeswoman said: "All incidents must be reported immediately and formal investigations are required in each case.
"Prisoners released in error are normally returned to custody by the police to continue to serve their sentence.
"None of those released in error during 2009 remain unlawfully at large."
Earlier this month, murder suspect Jason Bethell, 31, was accidentally freed from prison in Chelmsford, Essex.
Bethell, of Jaywick, Essex, who was on remand accused of murdering 58-year-old John Smith, later handed himself in to police.
Meanwhile, the Ministry of Justice says the extra prison places have been provided by building new wings, conversions and using existing units more efficiently.
A new prison, HMP Bure, opened on Monday. It was built on the site of RAF Coltishall in Norfolk and will be home to Category C sex offenders.
The government announced plans to take prison place numbers up from 81,000 to 96,000 by 2014, following a review of overcrowding in jails in 2007.
The report, by Labour peer Lord Carter, recommended a building programme after looking at the prison system and the supply and demand of prison places.
Justice Minister Maria Eagle said the government was pleased by the progress made since Lord Carter's report and "committed" to relieving prison population pressures.
The Ministry of Justice said plans include the provision of 264 prison places at an expansion of HMP Forest Bank in Greater Manchester, and 77 at HMP Bronzefield in Middlesex.
In April, Justice Secretary Jack Straw announced plans to create 7,500 new prison places by setting up new 1,500-place establishments.
Ms Eagle said: "We are delivering new prisons like HMP Bure and HMP Isis which will open next year next to Belmarsh. Plans to create 1,500-place prisons are in hand at Runwell in Essex, and Beam Park West, in Barking and Dagenham.
"These new prisons will help protect the public and make communities safer by providing regimes to punish and reform offenders."
Locking up more offenders for longer had contributed to cutting crime by 36% since 1997, she added.
In August, the Prison Reform Trust said almost two-thirds of prisons in England and Wales were overcrowded, with jails' intended numbers exceeded by almost 9,000.
It said overcrowding increases reoffending and called for prison to be reserved for serious and violent criminals.
Ministry of Justice figures put the prison population in July at 83,962 - 8,865 above the normal accommodation level of 75,097.