Two male staff members at the Yarl's Wood immigration centre in Bedfordshire have been fired for having sex with a female detainee.
A 23-year-old Roma woman reported having sex with the two men to police.
A third employee at the Serco-operated centre was also dismissed for failing to take any action when the female detainee reported the two men.
The news was announced by HM Chief Inspector of Prisons Nick Hardwick following a surprise inspection.
However, inspectors found no evidence that a "wider culture of victimisation or systematic abuse" had developed following the allegations of abuse at the 400-bed centre, he said.
Mr Hardwick said: "We were concerned to find that two staff had engaged in sexual activity with a female detainee, something that can never be less than abusive given the vulnerability of the detained population, and this staffs have rightly been dismissed."
Inspectors concluded that more female staff were needed urgently as there were not enough for a mainly-female establishment.
They also found that a number of women at the centre - where none of the detainees have been charged with an offence - were detained for long periods, including one for almost four years. In another place, the surprise inspection found pregnant women had been held without evidence of exceptional circumstances required to justify their captivity. One of the women had been admitted to hospital twice because of pregnancy-related complications.
Yarl's Wood still holds detainees in the middle of a distressing and difficult experience and more thought needs to be given to meeting their emotional and practical needs. For the most vulnerable of the women held, the decision to detain them appears much too casual.