The government is to seek private sector investment in the management of 50,000 temporary NHS staff, prompting fears about privatisation of the health service.
Karen Jennings, the Unison union's head of health, told politics.co.uk plans by the Department of Health (DoH) to explore investment opportunities at NHS Professionals were "purely about Tory plans to promote privatisation".
The government insisted the purpose of the move was to save the NHS money, however, and that it viewed the body in question as a "business, not a public service".
NHS Professionals manages a pool of 50,000 workers and is currently wholly owned by the government. It provides workers for about two million shifts a year for NHS organisations which need extra labour.
"The department intends to hold meetings for discussion with the private sector on potential options and opportunities to invest in NHS Professionals," the DoH said in a statement released this morning.
"Parties interested in this opportunity are encouraged to contact the Department of Health."
Ms Jennings said the move risked undermining the original purpose of NHS Professionals - "because private agencies were ripping off hospitals by charging them outrageous fees for recruiting or finding staff for shifts".
She added: "It makes no sense at all to bring back private companies who will want their slice of the action in return.
"This proposal is purely about Tory plans to promote privatisation and hive off parts of the NHS to private companies, regardless of the consequences on patient care.
"At a time when hospitals are being forced to make huge savings pushing up staffing costs unnecessarily is very damaging."
Several private sector companies already provide temporary staff to hospitals in the same way that agencies provide flexible staff elsewhere in the public sector, however.
Ed Burns, of consultancy firm Newton which has helped over ten NHS trusts deliver efficiency savings, said most used a number of companies to provide their 'bank' cover.
"Depending on how it is run, it could make cost-cutting easier or harder," he told politics.co.uk.
"Using temporary staff is always going to be more expensive than providing internal flexibility. However it is only ever sensible to provide a certain amount of cover internally - and this is only usually focused on nursing and admin/clerical staff.
"Getting manning levels right and having visibility and flexibility across a whole trust and not merely within departments would have advantages regardless of how a hospital is using temporary staff."
Mr Burns argued that companies providing lower-cost solutions for hospitals would help lower market rates and save hospitals money.
Getting the balance right between permanent and flexible staff offered much greater scope for real savings, however.
He added: "It's also vital that their HR departments put in place contractual arrangements to ensure their staff provide a minimum amount of overtime before being able to work with private sector bank agencies."
A DoH spokesperson said the government wanted to explore ways in which the commercial skills of the independent sector could make NHS Professionals more efficient.
She insisted the body was "not an agency". "It is an outsource service provider, and its primary role is to manage the flexible staff bank for NHS Trusts.
"NHS Professionals is often able to manage the flexible staff bank more efficiently and effectively than NHS Trusts managing it themselves, so it reduces the cost of running the staff bank, and helps organisations reduce their dependence on external agencies.
"NHS Professionals will only thrive if it continues to outperform NHS Trusts managing the service themselves. We want to discuss options with potential independent sector investors that could help to achieve this, and ultimately improve services outcomes for patients."