Angry Hammersmith and Fulham Council staff say they refuse to have new working conditions imposed on them.
New contracts would slash maternity pay from 16 weeks at full pay to the statutory six weeks and cut so-called 'special leave,' for employees who are carers or who have dependents, from around 30 days to five days a year.
Unison branch secretary Noreen Morris said members had already fought off one attempt by the council to force employees to sign new contracts, after Hammersmith and Fulham Council withdrew proposals to dismiss staff who refused to do so.
"If the council attempts this again or tries to impose these conditions another way, we will ballot for strike action," she said. "Members are serious about action and they were unanimous in their support for the motion."
Staff are also concerned at proposed changes which would see them work evenings and weekends with no additional compensation and they called for plans to move the council's contact centre to Rochdale, near Manchester, to be scrapped.
Standard working hours are to be extended from 10am to 4pm, under the proposals, which would make working parents lives difficult, the branch secretary said.
Days before a meeting of 450 Unison members last Tuesday, Hammersmith Council withdrew threats to dismiss staff who did not accept agreeing to the new terms. It said it was trying to reduce costs, improve services and bring its terms and conditions in line with other councils and similar organisations.
A spokeswoman said: "Some had concerns that the notice we issued showed that management had already decided to enforce the current proposals by dismissing and re-engaging staff. This is not the case. We want to listen to employees on the proposed changes and as a gesture of good faith we have withdrawn the notice."