The suspended farmworkers' strike in the Western Cape could re-ignite because of farmers firing workers, Cosatu said on Thursday.
The provincial Congress of SA Trade Unions said farmers in De Doorns were taking "punitive action" against workers for saying they could not sustain their families on R69 a day.
"With the calling off of the strike, the entire country breathed a sigh of relief, but farmers have refused to take back workers who were party to the protest," provincial secretary Tony Ehrenreich claimed in a statement.
Ehrenreich said renewed action would flare up in De Doorns on Friday and it could spread to nearby towns.
Cosatu announced on Tuesday that the strike had been called off, but said it would co-ordinate "the mother of all strikes against bad farmers" later in the year.
Farmworkers went on strike last year, demanding that the minimum daily wage be increased from R69 to R150, and that a cohesive land reform programme be implemented.
The strike was suspended on condition that Agri SA committed to "local-level" agreements and undertook not to victimise workers.
Agri SA president Johannes Möller was not immediately available for comment on the possibility of renewed strike action, or to confirm that workers had been fired.
Cosatu called on Labour Minister Mildred Oliphant to intervene.
"Should this matter not be resolved over the weekend, there is a bleak forecast for Monday, is all the workers would say," Ehrenreich said.
"Cosatu will be filing a section 77 notice against these farmers and will call [for a] boycott of grapes in the supermarkets, and also call on workers not to handle the grapes in any areas."
Section 77 of the Labour Relations Act relates to "protest action to promote or defend socio-economic interests of workers".