This is the latest of a series of stoppages that have damaged the economy.
Numsa, which represents over 220 000 metal workers and other artisans, said in a statement that the strike over wage demands, that will hit sectors including engineering, communications and automotive components, was “inevitable.”
“The national executive committee has agreed to the decision from our members to embark on an indefinite strike action, beginning on July 1,” deputy general secretary Karl Cloete said in Johannesburg on Thursday.
The union’s demands include a 12% wage increase in a one-year bargaining agreement.
Cloete said the union initially wanted 15% but subsequently revised this to 12%.
“[There will be] no settlement whatsoever unless a double digit increase is achieved,” he said.
Numsa said companies that would be impacted include Bell Equipment and industrial group Dorbyl.
On Wednesday, the Steel and Engineering Industries Federation of SA (Seifsa) said three months of negotiations had failed to produce a settlement. A deadlock in the talks was declared last month.
Last week Seifsa tabled a three-year wage settlement offer of between 7% and 8% for different levels of workers in the first year, and CPI-linked increases for 2015 and 2016.
The sector’s previous agreement, a three-year deal signed in 2011, would expire on June 30.
Meanwhile, the government will have negotiations with all stakeholders in a bid to prevent the strike.
“We are going to support all the affected parties to make sure this strike doesn’t take place,” said Communications Minister Faith Muthambi.
Numsa also said on Thursday that 11 000 members at Eskom would be willing to risk a wildcat strike, separate from its other national action, but gave no time frame for any potential stoppage.
Workers at Eskom are generally not allowed to strike by law because they are considered to provide essential services.
A downing of tools there by Numsa members could hamper the utility’s ability to keep the lights on, already a daily battle because of razor-thin margins between power supply and demand.
A four-week strike in 2013 by more than 30 000 Numsa members at major auto makers cost the industry around $2bn in lost output.
SA is still reeling from a five-month strike in the platinum mining sector that ended this week, which also pushed the economy into a contraction in the first quarter.
There are also concerns that the economy faces more labour strife.