Some foreign national prisoners are being held in jail, including in Dorchester, for several years awaiting deportation, it has been revealed.
Dame Anne Owers, the chief inspector of prisons, criticised "slow progress" on immigration cases of foreign criminals who have finished their sentences.
A quarter of prisoners Dorchester are foreign nationals whose sentences have been completed but who have not been returned, her annual report revealed.
She said: "We still find people who have been held in prison for a long time after sentence. On rare occasions it can be years, but it has certainly been improved. There has been a considerable focus on it and more prisoners being put into immigration detention."
Her report added: "There were still considerable frustrations arising from slow progress on immigration cases, and the issuing of authority to detain forms just before the end of sentence."
Figures released in Parliament showed that last year around 550 foreign prisoners were kept beyond the end of their sentence in the average month. Another 1,250 are held in immigration detention centres. Around a fifth are thought to have spent more than a year in custody after finishing their jail term. Each inmate costs an average of around £38,000 a year.
Immigration minister Phil Woolas said: "Public protection is our top priority - that is why we keep those deemed to be a risk to the public in prison while we seek to remove them from the UK.
"Those that break the rules will not be tolerated which is why we are removing record numbers of foreign national prisoners. We've had considerable success working with foreign governments to establish re-documentation procedures - helping us speed up the removal of prisoners."
Shadow justice secretary Dominic Grieve said: "One reason this Government has released 80,000 criminals from prison early is because they have failed to deport so many foreign national prisoners. This woeful incompetence has put the public at unnecessary risk."
Liberal Democrat home affairs spokesman Chris Huhne said: "Ministers love to brag about their record on deportations but Labour has failed to deport dangerous criminals and has focused on easy targets like families in a bid to boost numbers."Reuse content