Two staff at a privately-run immigration removal centre for women have been fired for engaging in sexual activity with a detainee.
A third employee at Serco-operated Yarl's Wood, in Bedfordshire, was also sacked for failing to take any action when the female detainee reported the two men, HM Chief Inspector of Prisons Nick Hardwick said.
It was reported last month that police were investigating claims that a 23-year-old Roma woman who was held at Yarl's Wood was subject to inappropriate sexual behaviour from guards.
But inspectors found no evidence that a "wider culture of victimisation or systematic abuse" had developed following the new allegations of abuse at the 400-bed centre.
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 these staff had rightly been dismissed."
Mr Hardwick added: "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 itself appears much too casual."
Yarl's Wood holds mainly single adult women but also holds a number of adult families and there is a short-term holding facility for adult men.
Inspectors concluded that more female staff were needed urgently as there were not enough for a mainly women's establishment.
A number of women at the centre - where none of the detainees have been charged with an offence or held through normal judicial circumstances - were detained for long periods, including one for almost four years.
Elsewhere, 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.
And detainees who had clear human trafficking indicators - such as one woman who had been picked up in a brothel - had not been referred to the national trafficking referral mechanism, as required.
Refugee Council women's advocacy and influencing officer Anna Musgrave said: "Some of the findings of this inspection are shocking.
"Women in immigration detention are extremely vulnerable, with many likely to be victims of gender-based violence, so we're horrified to hear that male officers enter women's rooms without permission.
"It's particularly disturbing that officials are not even following current policy and pregnant women are being detained without any clear reason.
"Pregnant women with insecure immigration status already have high-risk pregnancies and we believe they should not be detained under any circumstances. There is absolutely no excuse for compromising the health and well-being of a mother and her baby.
"This report shows that urgent changes are needed at Yarl's Wood to ensure that vulnerable women feel safe and that their dignity is respected."
Rachel Robinson, policy officer for Liberty, said: "Revelations of sexual abuse and the unjustifiable detention of vulnerable women still cast a dark shadow over Yarl's Wood.
"Attempts to avoid scrutiny and challenge via cuts to legal aid and the nasty Immigration Bill would deny more victims a voice and leave the Government that bit freer to act with impunity."
John Tolland, Serco's contract director, said: "We are really pleased that this inspection report recognises the improvements Serco has made at Yarl's Wood and considers it to be an establishment where residents feel safe and there is little violence.
"Our managers and staff have worked hard to establish and maintain good relationships with the residents, who are vulnerable people in the middle of a distressing and difficult experience.
"However, we are not complacent. As the HMCIP report says, we need to make further improvements and we are already working closely with the Home Office to implement their recommendations."
Shadow home secretary Yvette Cooper said: " The evidence of abuse at Yarl's Wood is appalling. The Home Office and Serco have a responsibility to act much faster and much more effectively to stamp out abuse and make sure vulnerable women get the support and help they need.
"Yarl's Wood is improving much too slowly.
"The Home Office has still not told us how long it knew abuse was taking place at Yarl's Wood. Or why it is still failing to spot the signs of trafficking or of mental illness.
"The Home Office cannot shirk responsibility. Serco may run the centre but it is up to the Home Office to make sure people are being treated humanely, with proper procedures and training in place.
"I called on the Home Secretary last month to get the independent UKBA inspectorate to review urgently the operation of outsourced centres run by private contractors such as Serco and we have heard nothing.
"This report shows the Home Office are failing in their duties and the Home Secretary needs to put that right immediately.
"Our immigration system must be efficient, effective and beyond reproach - especially in how it deals with vulnerable people. There cannot be any place for abuse anywhere within that system."
A Home Office spokesman said: "Detention is a vital tool that helps us remove those with no right to be in the country, but it is essential that our facilities are well run, safe and secure.
"Safeguarding those in our care is our utmost priority and misconduct is dealt with swiftly and robustly.
"We are carefully considering the contents of the report and will respond to each of its recommendations in due course."