The growing toll of mental health problems within the armed forces has led to a bill of more than £9m from the Priory Group for the Ministry of Defence.
The clinics are world famous for treating film stars and music celebrities for addiction but they are also privately treating hundreds of servicemen and women for mental health problems.
Figures revealed yesterday in written answers to the Commons showed that the MoD paid £4.1m in 2004-5 for the treatment of 321 servicemen and women and a further £4.4m for 351 troops in 2005-6.
The figures highlight the toll the Iraq war, service in Afghanistan and security operations across the world are exacting on the armed forces. It follows reports that members of the Territorial Army, reservists, who have served in Iraq are suffering more mental health problems than their professional comrades-in-arms.
Reservists were found to be at 25 times greater risk of having common mental health symptoms and service-related fatigue and 50 per cent more likely to suffer from post-traumatic stress disorder.
The Priory Group became embroiled in controversy when the head of the company, Dr Chai Patel, was blocked for a peerage by the Lords Appointments Commission after being nominated by Tony Blair. Dr Patel was among Labour Party supporters who had given secret loans to the party in the run-up to the general election. The Metropolitan Police are investigating criminal allegations against Mr Blair over the "cash-for-peerages" row, following a complain by Angus MacNeil, a Scottish Nationalist MP.
MPs suspended a parliamentary inquiry into the peerages dispute last week after the Met's chief investigator on the case, John Yates, warned it could undermine their criminal investigation. Mr Yates has been told to produce results by July or the MPs will resume interviews with key witnesses, including Lord Levy, Labour's chief fund-raiser.
Mr MacNeil tabled the question to the MoD asking how much it had paid the Priory Group for psychiatric services since it won the contract in 2003. Tom Watson, a Defence minister, said at the time the contract was awarded, NHS hospitals did not bid for it. It was previously carried out by the Duchess of Kent Pyschiatric Hospital, which cost the MoD £10m in 2002-3.
A spokesman for the MoD said: "We have these contracts with the Priory Group and do not use the NHS because we need to give a guarantee that there will be a bed when they are needed."
He said the private clinics ensured that soldiers could be treated near to their homes, although they were all in-patient cases. Mr Watson said community-based mental care was available to every military unit through centres in the UK and satellite units in Germany and Cyprus.
In a separate reply to Mr MacNeil, Mr Watson said the MoD contract for an in-patient psychiatric healthcare service was subject to competitive tender.
"The Priory Group's tender best met our requirements for high quality in-patient care, without delay and on a regional basis, and they were awarded the contract... Whilst NHS providers did have the opportunity to bid, there were no expressions of interest from the NHS."Reuse content