Members of two unions, the GMB and Unite, have voted against a pay deal, raising the prospect of further strike action by council staff.
Last week the offer, worth 3% this year and 2.5% next year, was accepted by the other main union Unison.
The GMB said its officials would decide their next move at a meeting next week.
Local authorities across the country have already been hit by two one-day strikes earlier this year, which caused widespread disruption to services.
GMB members have voted almost 2-1 against the latest Cosla pay deal.
'Spoken clearly'
Alex McLuckie, the GMB's senior Scottish organiser, said: "GMB members have spoken clearly and this will be paramount when deciding our next steps."
The GMB represents 25,000 council workers in Scotland.
The Cosla offer was worth 3% this year and 2.5% in 2009 and was accepted by members of Unison, Scotland's biggest union, last Wednesday.
Unison leaders had urged its membership to reject the offer, claiming it was not a good deal.
Unite senior official Jimmy Farrelly said his union's members voted for rejection in a high turnout.
"There is a genuine resentment and frustration by our members which stems from the employers originally stating that the offer would not be improved on the basis that it was fair and all they could afford," he said.
He added: "We believe more needs to be done on the part of the employers and the Scottish Government to end this damaging dispute."
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