Around 400 builders at one of the 2010 South Africa World Cup stadiums have been dismissed for going on strike.
Most of the workers are members of the National Union of Mine Workers (NUM).
"They have been on illegal strike and were sacked on Tuesday," said George Ledwaba, the union's local spokesman.
"If this strike, which began on 6 February, goes on for another week, it will affect the scheduled completion date of the stadium," he said.
"Our target completion time is July, we will probably have to extend this deadline as a result of the strike," the stadium construction manager, Neil Fourie, said.
He said that the workers downed tools over wages and bonuses.
Ledwaba condemned the strike by his union members which he said was "illegal" and "irresponsible" because, according to him, they did not follow due process before downing tools.
About 1,000 workers are involved in the construction of the stadium.
Workers at the 46,000-seat Mbombela stadium have staged earlier strikes, most significantly in February last year to demand a near doubling of their wages.
Similar disputes have previously broken out at stadiums in Durban and Cape Town, but World Cup organisers say they are confident those projects will be completed on time.
Ten stadiums in nine cities will play host to the 2010 World Cup, the first time the tournament has been staged in Africa.
The Fifa deadline for the completion of all the stadiums is 15 October.