The Economic Freedom Fighters said the Labour Department has officially given them the go-ahead to start their own union - the National Trade Union Congress.
Barely eight months after it was formed, the EFF managed to garner 6.35% of the votes in the national elections in May.
An EFF spokesman said yesterday party members could not belong to Cosatu-affiliated unions.
According to an EFF insider, the party submitted registration forms to the Labour Department on May 27 and received its registration number yesterday.
The inside source said the EFF would travel to all nine provinces to recruit people into their union.
"We already have a huge number of registered members but we could not approach employers for recognition without registration," he said.
The union could prove stiff competition to Cosatu, which is battling internal divisions. The biggest split has been with The National Union of Metalworkers of SA, which has just called for a strike.
The EFF union plans to target marginalised workers such as petrol attendants, domestic workers, farmworkers, security guards and workers in the retail sector.
Eddie Mathiba, the union's interim general secretary, said: "The time is now for the economic emancipation of the proletariat.
"Gone are the days of capitalist vehicles driven by employers and Cosatu affiliates," he said.
Many EFF members believe that Cosatu's alliance with the ANC has made it toothless, and that the trade union federation no longer strives for workers' rights but rather keeping the fat cats in power.
The EFF source said: "[Cosatu] has become an ANC electioneering tool, which only remembers the workers when it has to campaign for the ruling party."
He added that many workers had no one who truly strove to protect them from exploitation and unfair dismissals and that existing unions have betrayed them.