Firm forced to disclose secret internal report as Keith Vaz says he is 'shocked' by events at immigration detention centre

Serco, the private outsourcing giant, is to be investigated by MPs after it was forced to disclose a secret internal report revealing evidence that it failed to properly investigate a claim of repeated sexual assaults by one of its staff against a female resident at Yarl's Wood immigration detention centre.

The document, which was marked confidential, was made public last week following a four-month legal battle between Serco and Guardian News and Media. Lawyers said the report demonstrates a culture of disbelief towards women inside the detention centre, which is run by Serco, and hailed the high court's decision forcing Serco to disclose the document as a victory for greater transparency.

The revelation comes a day after it was disclosed that Serco could be among companies to take over the running of privatised children's social services, including child protection, under proposals being considered by Michael Gove's Department for Education.

Keith Vaz, chair of the home affairs select committee, said the report's revelations were "shocking" and warned he would be summoning senior Serco figures to parliament next month for them to explain their actions.

The report details an investigation into the claims of a 29-year-old woman from Pakistan that she was sexually assaulted three times at Yarl's Wood by a Serco health worker between November 2010 and January 2011.

Although the claims of Sana (not her real name) remain unsubstantiated after investigations by police and the Home Office, the report's findings and process have angered MPs and lawyers.

Among them is the revelation that a Serco guard who appeared to believe the claims made by the alleged victim be given "guidance" to assist her "objectivity" in future, and that Serco believed the alleged victim lacked credibility because her allegations were deemed too consistent and detailed.

Vaz said: "These are shocking revelations and they demonstrate to me that an internal investigation is not enough. 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.

"Serco will need to appear before the committee at our next inquiry looking at the whole procurement process, which we will be conducting very shortly."

The gates of Yarl's Wood asylum centre, which is run by Serco. Photograph: Bryn Lennon/Getty Images

The Labour MP said that until he felt all the information on the issue had been collated, Serco should be prevented from bidding for fresh contracts.

"We should not be giving out contracts to companies who have let the country down," he said. "This is clearly a case where there are serious questions that remain to be answered."

Elsewhere, pressure mounted on the UK government to adequately scrutinise the ability of private companies to look after vulnerable individuals. Rashida Majoo, the United Nations special rapporteur into violence against women who was banned by the Home Office from entering Yarl's Wood last month, said that the new material would be examined and potentially forwarded to other groups, including the UN special rapporteur on torture.

The developments will intensify concern over the outsourcing of contracts to look after some of the most needy people in society. On Saturday the Guardian revealed plans by Gove's department to consider outsourcing child protection services to companies such as Serco.

Experts and lawyers said that the contents of the internal report had prompted fresh alarm over whether Serco could be trusted to handle such complex issues.

Former director of public prosecutions Ken Macdonald QC said that when commercial organisations are given the right to run state contracts for profit, "it is essential that they show they are capable of acting in the public interest. Here, the suggestion is that in the face of credible allegations that a member of their staff had committed repeated sexual assaults on an especially vulnerable inmate, Serco conducted an inadequate investigation in secret and then did everything they could to hide their findings from the public."

Harriet Wistrich, the lawyer representing Sana, said: "The failure of the investigatory process affects not only the complainant but all women detained at Yarl's Wood, since it allows abuse to continue and thrive."

Serco's report says:

■ The company believes that the female detention officer who believed Sana's account should "be given advice and guidance to assist her in being more objective in the future. She should not be making judgments without knowing all the facts."

■ The same guard "appears to have believed the allegations without considering the possibility that they have been fabricated."

■ Serco investigators concluded it was possible that Sana "is being advised by her solicitor and befriender of actions to take in order to thwart her removal directions".

■ Comments made by an investigating police officer suggest [Sana's] answers were "very clinical and she didn't appear distressed".

■ The alleged perpetrator is a "family man with strong religious beliefs and would have a lot to lose".

■ It is possible that Sana "manipulated the situation in order to put [alleged perpetrator] in a position where she could make an allegation".

Norman Abusin, Serco's director at Yarl's Wood, 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."

• The hunt for the truth about Yarl's Wood asylum centre