Serco faces fresh allegations over its running of Yarl's Wood detention centre

Outsourcing company is again in the spotlight after accusations of charging for mental health assessments that did not occur

The National Audit Office (NAO) is looking into fresh allegations over the way a private sector company has run Yarl's Wood, the UK's largest detention centre for women facing deportation.

Parliament's independent financial watchdog has been alerted to allegations that Serco, the government contractor that has managed the facility since 2007, has inflated certain figures and failed to carry out mental health assessments for groups of asylum seekers. The NAO is understood to have just started preliminary discussions with some of those who have made the claims and will soon decide whether to launch a full inquiry.

Yarl's Wood, in Bedfordshire, which can hold around 400 women and families at any one time, has raised a number of concerns in recent years: in 2013 a Roma woman accused officers of "taking advantage of the girls that were detained", and asserted that sexual contact was not consensual; and this year it was claimed that a UN official was blocked from examining conditions in Yarl's Wood.

A source close to the initial investigation said there were suggestions that people were "referred out of the country without proper mental health assessments". It is understood that there are allegations that mental health assessment figures were "manipulated", the suggestion being that Serco would receive more money for carrying out more of these costly evaluations. The NAO's job is to make sure that taxpayers' money is spent correctly.

A fully-fledged investigation could prove damaging for Serco, which is led by Sir Winston Churchill's grandson, Rupert Soames, as it has been involved in a number of scandals since last summer.

The company was found to have charged the taxpayer for tagging non-existent and dead offenders, for which it had to repay the Ministry of Justice £68.5m last December.

As a result of the scandal, Serco fell out of the blue-chip FTSE 100 list of the country's most valuable companies listed on the London Stock Exchange, and has undertaken a programme of what it describes as "corporate renewal".

Chris Hyman, the long-serving chief executive, quit late last year and Mr Soames, who is also the brother of Conservative grandee Sir Nicholas Soames, left power equipment supplier Aggreko to lead Serco out of its mire. However, Serco shares are still well down on their value at the start of the year.

A spokesman for the NAO said: "We're looking at the evidence … and deciding which course of action to take, if any."

Alan Stannard, Serco's account director for Home Office work, said that most recommendations for improvement published in previous reports into Yarl's Wood have been put in place: "The NAO has not approached Serco about Yarl's Wood, but we would, as always, fully cooperate with any request from them. Yarl's Wood is regularly subject to scrutiny by a variety of independent bodies, including the Care Quality Commission, Her Majesty's Inspectorate of Prisons and the Independent Monitoring Board."

Yarl's Wood opened in 2001 but was handed to Serco in a deal worth £85m seven years ago. This form of "outsourcing" traditionally state-run services has become increasingly prevalent as successive governments hope that the introduction of greater commercial nous will help reduce costs.

The coalition has spent £88bn on outsourcing, according to figures from the Information Services Group earlier this year. This was nearly double the £45bn spent by Labour in its last four years of government, and means that tens of thousands of public sector staff have transferred to the private sector as a result.

Parliament's powerful Public Accounts Committee, chaired by Labour's Margaret Hodge, often follows up NAO inquiries with its own investigations.