South Africa’s National Union of Mineworkers (NUM) said on Tuesday its coal workers will embark on a strike if the Chamber of Mines does not meet its wage demands.

A strike in the coal sector could affect power supply as the lion’s share of electricity in Africa’s most industrialised economy is generated from coal.

The mining sector has in the past been marred by labour unrest that cost it billions of dollars in lost output.

The union said it has given the employers’ body, the Chamber of Mines, until Wednesday to change their current wage offer in the sector.

“We will declare a strike and that is going to have devastating economic consequences,” NUM spokesman Livhuwani Mammburu said.

The Chamber, which represents firms such as Anglo American Coal, Exxaro and Glencore, said it would issue a statement on the wage stand-off later on Tuesday.

The wage offers vary depending on the company, ranging from a 5 percent to 7.5 percent increase in 2018 and in 2019 for the lowest paid workers. Some of the mining companies are also proposing once-off payments as part of the wage offers.

The union is demanding a once-off payment of 1,100 rand ($78) this year and an 8 percent increase for 2018.

NUM’s demands also include a 9 percent wage increase in 2019, but the union said it will only sign an agreement for the third year if the Chamber accepts to remain in the collective bargaining forum.

The Chamber and unions agreed in June to negotiate wages collectively as an industry for 2017 instead of on a company-by-company basis, defusing a potential source of friction after NUM threatened to go on strike.

Much of the sector is still in the hands of the white-minority more than two decades after demise of apartheid, with black people comprising the bulk of the labour force.

($1 = 14.0775 rand)