A privately-run immigration removal centre for women failed to properly investigate allegations of sexual assault against a female resident by one of its members of staff, it has been claimed.
An investigation was carried out at Serco-operated Yarl's Wood, in Bedfordshire, after the claims relating to a 29-year-old woman from Pakistan came to light, but now the outsourcing company has been forced to disclose an internal report which reveals evidence that it failed to properly probe the claims.
The document, which was made public following a legal battle between Serco and Guardian News & Media, said another employee who appeared to believe the claims made by the alleged victim should be given "guidance" to assist her "objectivity" in future, and that Serco believed the alleged victim lacked credibility because her allegations were too consistent and detailed.
The claims by the woman that she was sexually assaulted three times by a Serco health worker between November 2010 and January 2011 were investigated by police and the Home Office, but not taken any further.
Serco's report also suggested it was possible that the woman was being advised by her solicitor of "actions to take in order to thwart her removal directions", while it was claimed that her alleged attacker was a family man with strong religious views and would therefore have much to lose.
Keith Vaz, chairman of the home affairs select committee, called for an external investigation into the "shocking revelations" and said the company should appear before the committee.
He told the Observer: "It's clearly the tip of the iceberg as far as these allegations are concerned and the way Serco has dealt with them.
"There needs to be an external examination following these revelations, which will look at the entire history of the allegations but also look carefully at the way in which Serco manage other properties of this kind."
It has emerged that Serco could also be among companies given the chance to bid to take over the running of privatised children's social services, including child protection.
Yarl's Wood, which holds mainly single adult women but also holds a number of adult families along with a short-term holding facility for adult men, has previously been subjected to heavy criticism by human rights campaigners amid reports of sexual misconduct by staff, women being detained for long periods of time and pregnant detainees being held without justification.
Last October it emerged that two employees had been fired for engaging in sexual activity with a 23-year-old detainee while a third was sacked for failing to take any action after the woman reported the two men.
A surprise inspection last year found 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, and inspectors found that 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.
In March Jamaican detainee Christine Case, 40, died in the centre after suffering what is thought to have been a heart attack.
Serco's director at Yarl's Wood, Norman Abusin, said: "Sexual contact between residents and staff is always completely unacceptable.
"We view any complaint of this type of behaviour extremely seriously and have strict procedures for dealing with any such complaints: they are always investigated and the necessary disciplinary action is always taken including informing the relevant authorities.
"Our managers and staff work hard to establish and maintain good relationships with the residents. The most recent HM Chief Inspector of Prisons inspection report, issued in October 2013, considered it to be an establishment where residents feel safe and there is little violence. However, we are not complacent and make continuous improvements to the services we provide."