The “prolonged detention” of foreign detainees in jail under immigration powers because of the department’s “inefficiencies” is “inexcusable”, said chief inspector of prisons Charlie Taylor.
A review of conditions for immigration detainees found many were in prison for “long periods with little or no progress in their cases being made by the Home Office”.
One woman told inspectors she had been waiting six weeks to be removed from the country after agreeing to return home and offering to pay for her own flights.
She said she had received no clear update on when she should expect to travel.
In other cases, the Home Office’s independent case progression panel had recommended some detainees be released but they remained behind bars in prisons already under “enormous strain”, the findings showed.
If there is no prospect of removal within a reasonable period, detention “ceases to be lawful”, the watchdog’s report said.
But there was “little prospect of removal within a reasonable period for many detainees”.
Inspectors described the case of a person who had been detained for close to three years as “unacceptable”.
Migrant crossings on the English Channel surge amid heatwave - in pictures
“The prolonged detention of people under immigration powers, especially when it is because of inefficiencies in Home Office case-working procedures, is inexcusable given that so many prisons are already overcrowded,” Mr Taylor said.
Inspectors visited eight prisons and interviewed 45 immigration detainees and 33 staff in March and April.
"The progression of detainees’ immigration cases was not adequate in too many of the cases we reviewed, which meant that people were subject to avoidable periods of often lengthy immigration detention," their report said.
“This, combined with a lack of meaningful information from the Home Office regarding their case progression had led to frustration and feelings of helplessness in detainees, who told us they were often confused about what was happening.
“Slow case progression within the immigration system in general, and the restrictions on face-to-face contact during the pandemic, had compounded this problem.”
Nearly 700 migrants cross English Channel beating 2022 record - in pictures
Detainees struggled to obtain legal advice, with very few told they were allowed a free half-hour session with a lawyer.
Many prison and Home Office staff did not know of this entitlement, inspectors said.
Immigration papers were often served in English with no formal interpretation service to update prisoners on their cases, meaning inmates had to ask others to translate for them.
“Most worryingly”, vulnerable detainees, including victims of torture, were not routinely identified, the watchdog also found.
More than 200 migrants cross English Channel after pause in journeys - video
“Some of these people are extremely vulnerable," Mr Taylor said. "If these vulnerabilities are not monitored and addressed effectively, there is an increased risk that they will come to harm while in custody and that the integrity of the decision-making in their immigration cases will be undermined."
The Home Office has been contacted for comment.