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The city council has accused the union, which balloted its members on strike action against the outsourcing project, of putting jobs at risk by opposing the deal.
It said that CSL cancelled its bid to become a strategic partner in the creation of a joint venture company because Unison opposed the transfer of council staff to CSL.
CSL, however, told CW360 it had "reluctantly" decided not to proceed with its tender because it was unable to gain support for a transfer of council staff. The council, said CSL, appeared to favour secondment of staff, who would remain formally employed by the city council.
The outsourcing firm would not attribute blame to Unison, saying it dropped the bid only after failing to get "a clear endorsement from the council's leadership that a transfer of staff is the right way forward."
Unison said it had prevented the council from making a "grave mistake" by going into partnership with CSL.
Gill Hale, Unison northern region secretary, told CW360: "The reason we are opposed to CSL is because they don't deliver. Unison's aim is to deliver high quality public services and our experience of CSL is the opposite."
Doug Miller, senior consultant at outsourcing specialists Morgan Chambers, said the confusion over the Newcastle bid was typical of what can happen without good communication between all parties involved in an outsourcing deal.
"Communication is key," he told CW360. "Without communication you get insecurity and rumours start to spread. The whole situation could have been managed a lot better and it seems as if there were not enough ground rules set out from the start."
The council says it will press ahead with the creation of a joint venture company to run IT services, with British Telecom the favourite to take the contract.