Two Somerset councils have struck a multi-million pound deal with a private company to run some of their services.
The county council and Taunton Deane borough council say the move will save them millions of pounds.
Staff working in areas such as human resources, IT and finance will become employees of Southwest One, a company run by computer giant IBM.
Union officials claim the deal was struck in secret and was never fully debated.
Sensitive deal
Nigel Behan, from Unison, said: "This is about public accountability, it's about public sector jobs and also, it's about public services and council tax payers should know where and why this is happening."
County council chief executive Alan Jones said the sensitive nature of the deal meant there had to be an element of secrecy and he emphasised the benefits that would be achieved.
"Working together means we're a bigger market place, we have a greater buying power and working with IBM means that we can buy things such as energy," he said.
"We can buy information technology, computers and software, we can buy pretty much anything cheaper working that way."
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