The report examined deportations from Colnbrook Removal Centre and Heathrow
Illegal immigrants and foreign prisoners are failing to be removed from the UK because escort staff are "intimidating" them, a report has said.
The findings are revealed by the Chief Inspector of Prisons for England and Wales, Dame Anne Owers.
Inspectors monitored immigration escort staff involved in removing detainees from Heathrow Airport.
It says when officers escorting immigration offenders mistreat them, removals are more likely to fail.
The report cites the case of four Afghan detainees, who were initially willing to fly home.
Then, in what the report describes as an "unnecessary and intimidating" intervention, escort staff warned the detainees they would be "dealt with" if they were "naughty" on the plane. They refused to go.
Removals were also cancelled due to staff shortages and because nurses had not been booked to accompany detainees with health problems.
Sometimes detainees were flown back to the UK because their home countries said their paperwork was not in order.
On one occasion, escort staff taking a woman to Mozambique were themselves locked up after the authorities there questioned whether their documents were valid.
Dame Anne said: "This short report found worrying gaps and weaknesses in complaints and monitoring processes.
"It also found varying practice, with no evidence that the good and thoughtful approach of some staff was mirrored in clear and consistent standards of treatment, support and communication.
"This heightened the risk of ill-treatment or abuse, and was also likely to lead to failed removals."
David Wood, director of the criminality and detention group at the UK Border Agency said anyone involved in the detention and removal of detainees was highly trained.
"Many detainees refuse to leave the UK voluntarily, even when the courts say they must. In some cases individuals become violent toward themselves, the public or our staff when it is time to go home.
"Detainee escorts have a very difficult job to do in carrying out what the public expects of the UK Border Agency in enforcing our immigration laws."