Glen Burley, chief executive of the Foundation Group, comprising South Warwickshire Trust, George Eliot Hospital Trust, Worcestershire Acute Hospitals Trust and Wye Valley Trust, told the BBC that Thornbury Nursing Services was "profiteering" from an overstretched NHS.
Burley said one of the group's four hospitals was spending over £500,000 on staff provided by Thornbury, which is owned by Independent Clinical Services (ICS), itself controlled by Jersey based company Acacium.
The investigation found Thornbury charged almost £2,000 for a 12-hour bank shift by a specialist paediatric nurses and £91.45 per hour for a midweek afternoon and evening shift.
Burley said Thornbury was targeting areas where nurses were in short supply and tempting workers away from the NHS with higher rates.
Danny Mortimer, chief executive of NHS Employers (part of the NHS Confederation), said: ‘There has been mounting concern that a small number of staffing agencies are exploiting the pressures facing the NHS. The BBC raises serious questions about how much public money flows to these "last resort" agencies and their owners. The NHS is making real strides in retaining NHS staff, and reducing reliance on agency staff.'
Sir Julian Hartley, chief executive, NHS Providers, said: ‘Safety of patients and staff is paramount. Trusts work hard to make sure that staffing is adequate to provide safe, high-quality care.
‘That's a tough job with more than 110,000 NHS vacancies across England, stretched budgets and during months of strikes.
‘Trusts do everything they can to avoid unnecessary costs but with staff in short supply spending on agency staff will continue to be part of the picture.'
A spokesperson for Independent Clinical Services, said: ‘We provide a range of services to healthcare clients in the UK and the Republic of Ireland in both the private and public sectors.
‘We offer a valuable and flexible service that helps healthcare providers - both NHS and independent - guarantee safe staffing levels. We ensure the right nurses, with the right skills, are available to deliver safe care to patients. Requests are typically at very short notice often as little as four hours before a shift is due to start.
‘This service is not intended to provide long-term staffing solutions. The level of rates necessarily reflect the last-minute and urgent nature of requests and the requirement for highly skilled workers.
‘We recognise the strain the NHS and other organisations are under and are proactively working with customers with a large reliance on last-minute agency workers to lower their costs.'
A Department of Health and Social Care spokesperson said: ‘We are ending the use of expensive off-framework agencies - as announced in the Budget.
‘NHS staff are prioritised when shifts need filling, and we are controlling spending by limiting hourly pay for temporary staff.
‘While there are already a record number of NHS staff overall, the NHS Long Term Workforce Plan will see an unprecedented expansion in training and a significant boost in retention, reducing the need to hire agency workers.'
