The government has been accused of "promoting privatisation" of the NHS after a surprise announcement of sell-off plans for an agency which provides thousands of nurses and other health workers to hospitals across the country.

The department of health said it would explore options for private sector investment in the state-owned NHS Professionals, which has 50,000 workers on its books and places staff for two million shifts a year at 77 health trusts.

Unions were furious over the sell-off. Karen Jennings, UNISON Head of Health, said that "the whole reason that NHS Professionals was set up was because private agencies were ripping off hospitals by charging them outrageous fees for recruiting or finding staff for shifts.

"It makes no sense at all to bring back private companies who will want their slice of the action in return ... This is purely about promoting privatisation ...regardless of the consequences on patient care. A sell-off may bring a short-term cash injection, but cost NHS Trusts vast sums in the longer term."

However the department said the plan "is in line with the government's policy to maximize the value of assets and commercial opportunities," the department said in an e-mailed statement today. The department will hold meetings with the private sector on "options and opportunities," the statement said.

There are concerns that the move may be illegal. One website, chiefofficers.net, claimed that "[a] question arises as to whether this is a lawful activity under securities laws [governing the public sector in Europe]. The general principle is that it is illegal to seek investment in a company except in very limited circumstances. A public offering that does not fall within those exceptions will often result in serious penalty for those who are convicted of the various offences related to the conduct.."

Unison said said that this may be the "thin end of the privatisation wedge" as European rules exclude the NHS from having to contract out services – yet such a sell-off signals that the government is prepared to aggressively promote the private sector in the NHS.

The union said it was now contemplating a "judicial review" of the decision by NHS chief executive, Sir David Nicholson, to press ahead with reorganisation plans that have "neither parliamentary approval nor legal backing".

Nicholson wrote to NHS managers outlining a series of "actions that we need to start now" that may see wards shut and hospitals closed. Last week health secretary Andrew Lansley backed the move of specialist cancer surgery from Royal Cornwall Hospitals Trust to Plymouth Hospitals Trust.

"The Government's White Paper will change forever the NHS as we know it. These sweeping changes were not part of any party manifesto and it is outrageous that these changes are being brought in without consulting the public, patients, staff and unions," said Jennings. "In writing to NHS managers, Sir David is working on the premise that the consultation is only about the best way to achieve pre-determined outcomes – this makes it nothing more than a paper exercise and a sham.