The report by HM Inspectorate of Prisons revealed that many migrants remain behind bars while “little or no progress” in their cases is being made.
One detainee was found to have been locked up for an additional 33 months before a decision was made about their future.
Home Office figures from last year showed that 31 had been inside more a than year beyond the end of their sentence.
Chief Inspector of Prisons Charlie Taylor described that as “inexcusable”.
“If the Home Office started work on these cases earlier, then people could be processed out of the system far more quickly, and they wouldn’t have to wait around in prison beyond the end of their sentence, without having a clue about when they’re going to be let out or removed from the country,” he said.
The report found that those kept in prison rather than immigration detention were denied the same rights.
In a statement, a spokesperson for the government said: “Foreign criminals who remain in prison ahead of removal include dangerous individuals who have committed serious crimes.
“The government has improved access to legal advice and ensures we carefully consider a person’s vulnerabilities.”