The deal, due to be formalised at a signing ceremony at the end of this month, is understood to give Unison national recognition, covering all new local government contracts the company takes on.
CSL will sign a deal with the Public and Civil Service union at the same time.
Most union/contractor recognition agreements in local government are negotiated on a workplace basis.
Like its competitor Capita, CSL was established by CIPFA, but has been independent since 1987. In 1993 it was taken over by consultancy giant Deloitte Touche. The company now has around 2,500 staff and 17 council contracts, predominantly in revenue and benefits administration. This accounts for more than half of its business.
The imminent deal may prove controversial at Unison's local government conference next week.
Sheffield City Council's decision to contract its benefits service to CSL - the company's largest council contract - prompted a bitter stand-off between the council and union. Some benefits staff went on strike (LGC, 5 March).
When the council's former Labour leadership decided to grant CSL the
contract, it cited the Unison branch action as a factor in the decision (LGC, 1 April).
The 8,000 member strong Sheffield Unison branch was suspended last month over allegations that union funds were diverted to support radical left-wing political campaigns (LGC, 6 May).
The union is still investigating the claims, which are denied by branch officials.
Neither CSL or Unison would comment on the pending agreement.