A pregnant woman in a wheelchair was tipped up and had her feet held by staff as she was forcibly removed from the UK, inspectors have said.
G4S staff at Cedars centre near Gatwick used substantial force with significant risk to the unborn child, the Chief Inspector of Prisons said in a report.
Nick Hardwick said it was "simply not acceptable to initiate force for such purposes".
G4S said its staff were concerned that the woman risked causing herself harm.
Cedars pre-departure centre for families, which opened last year, replaced the unit at Yarl's Wood in Bedfordshire.
'Holding her feet'
G4S, which failed to lay on enough security staff for the London 2012 Olympic Games, provides security for the UK Border Agency (UKBA) at the centre, in Pease Pottage, West Sussex.
The pregnant woman was given a wheelchair to help her in the departures area.
When she resisted "substantial force" was used by G4S staff and the wheelchair "was tipped up with staff holding her feet".
"At one point she slipped down from the chair and the risk of injury to the unborn child was significant," the report said.
"There is no safe way to use force against a pregnant woman and to initiate it for the purpose of removal is to take an unacceptable risk."
Force was used against six out of the 39 families, including two children, at the centre, which holds families for up to a week, the report said.
'No safe way'
Mr Hardwick said: "We were very concerned to find that force had been used to effect the removal of a pregnant woman, using non-approved techniques.
"There is no safe way to do this while protecting the unborn child."
The pregnant woman's husband had been disruptive the night before, "shouting and kicking doors, causing some damage", the report found.
"At one point it was judged that he had been trying to separate healthcare staff offering to examine his wife to take them hostage," the report added.
"Staff were sufficiently concerned by his behaviour to take him to the 'cool down' separation room in full personal protection equipment before his removal."
The cool down room was "stark and not conducive to helping people calm down", the inspectors added.
Judith Dennis, of the Refugee Council, called for UKBA to heed the report's recommendations, which include that force should only ever be used against pregnant women and children to prevent harm.
Jerry Petherick, managing director of G4S custodial and detention services, said the welfare of people in its care was its top priority.
"In this incident, our staff were concerned that the woman risked causing herself harm and took the necessary steps to prevent this," he said.
"We will be examining how best to take forward the recommendations made following this incident, but it should be noted that the report praises staff for their exceptional level of care and the considerable steps they take to avoid the use of force."