The IUF union has accused Unilever of "double standards" in pay and benefits negotiations for workers in and outside of Europe post the McCormick deal.
It emerged in July the London-headquartered FMCG major had made a commitment to European employees to honour existing pay and benefits for two years after the completion of the tie-up with US-based McCormick & Co.
However, similar arrangements for staff outside of Europe are for a shorter one-year period, according to Switzerland-based IUF, which represents the food, drinks, agricultural and hospitality industries.
Unilever, which is listed in London, Amsterdam and New York, announced in March it was selling most of its food assets to McCormick for $44.8bn. The transaction is expected to close by the middle of next year but has already attracted the scrutiny of the UK's Competition and Markets Authority (CMA).
The IUF claims the "associated restructuring and consolidation have once again generated uncertainty and anxiety among workers across the business".
The union added in a statement: "Unilever has indicated that, outside Europe, transferred employees who remain with the new combined business will, as a baseline, receive ...