The 340,000-member National Union of Metalworkers of South Africa (NUMSA) draws its members from car manufacturing, the metal industry, transport and general workers.
The strike would be the latest in a string of work stoppages in Africa's largest economy, weeks before a May 7 general election."It is a strike that members in all other unions can join. We think half a million workers will take part," Karl Cloete, deputy general secretary of NUMSA, told Reuters on Sunday.
In December, NUMSA, the biggest bloc in the COSATU labour grouping, said it would not support the ruling African National Congress in the election.
Many South Africans are disgruntled by the slow pace at which the ANC led by President Jacob Zuma is rolling back poverty two decades after the end of apartheid.
A strike over wages in the platinum sector has been going on for nearly two months and has cost employees more than 3.8 billion rand ($354 million) in lost earnings.
The strike has cost mining companies 8.6 billion rand in revenue so far, according to a tally updated almost every second on the Chamber of Mines' website.