A pregnant woman in a wheelchair was tipped up and had her feet held by staff from the firm behind the Olympics security shambles as she was forcibly removed from the country, inspectors have said.
G4S staff used substantial force and unofficial techniques and the "risk of injury to the unborn child was significant", the first report on a new pre-departure centre used to remove families from the UK found.
Nick Hardwick, the Chief Inspector of Prisons, said it was "simply not acceptable to initiate force for such purposes".
The pregnant woman facing removal from the UK was given a wheelchair to assist her in the departures area of the Cedars centre in Pease Pottage, West Sussex, inspectors said.
But 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 of the 39 families, including two children, going through the centre which holds families for up to a week before their removal.
It has largely replaced the detention of children in the Yarl's Wood immigration removal centre and families are offered support by the children's charity Barnardo's while security is provided by G4S.
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 and it is simply not acceptable to initiate force for such purposes."
The pregnant woman's husband had been disruptive the night before his family's planned removal from Cedars, "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 so-called "cool down" room was "stark and not conducive to helping people calm down", the inspectors added.
Judith Dennis, of the Refugee Council, said the case was "shocking" and called for the UK Border Agency (UKBA) to heed the report's recommendations, which include that force should only ever be used against pregnant women and children "in order to prevent harm".
A G4S spokesman confirmed its staff were involved in incident.
Jerry Petherick, managing director of G4S custodial and detention services, said: "In every secure centre managed by G4S, the welfare of the people in our care is our top priority.
"In this incident, our staff were concerned that the woman risked causing herself harm and took the necessary steps to prevent this.
"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."
A UKBA spokesman said: "Returning families with no right to be in the UK is one of the most difficult and sensitive aspects of our work and we are committed to treating everyone in our care with respect and humanity.
"We will consider the recommendations in the HMCIP's report carefully and respond in due course."