The row comes after the council carried out a pay and grading review which has left some workers facing pay cuts of as much as 10,000. Around 80 per cent of staff, however, will not be affected.
A Unison spokesman said: "There is absolutely no reason for the council to take this action. They could have just allowed staff to turn up for work on or shortly after April 1 and work under the new arrangements without the threat of the sack."
The union said it would be seeking an urgent meeting with council chiefs and added it could not rule out balloting its members for industrial action.
But the council has dismissed the allegations as "absolute nonsense".
"The council is not in the business of sacking employees," it said. "This is not our intention and never has been.
"Currently 13,970 employees have signed up to the new terms and conditions. However, there is a small proportion...who have not yet told us of their intentions. All employees have to do is to turn up for work as usual on April 1 and acknowledge that they are working to new terms and conditions as offered to them. There is only an issue if employees refuse this offer."