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."
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