The public spat between former African National Congress (ANC) treasurer Mathews Phosa and President Jacob Zuma's son is getting more intense as both men continue to trade insults.

In the latest bout, Edward Zuma called Phosa a traitor after Phosa called him a "piece of vomit".

"Mr Phosa anger or not, insults or praises, you are a traitor," Zuma wrote in an open letter on Wednesday.

"He wants to service his long-time master, Johann Rupert," he said of Phosa.

The ongoing feud follows an opinion piece Phosa wrote in the Sunday Independent and the Daily Maverick calling for the president to resign.

"My plea remains: Please, for once, serve your people, and go. Go now. If you don't, history will judge you to be the chief architect of the destruction of the ANC," Phosa wrote.

In an open letter on Monday, the president's son accused Phosa of selling out to white monopoly capital.

"He has guts and no shame to want to mislead the people of this country in their quest for selfish and narrow interests of white monopoly capital," Zuma said.

In an interview with News24, Phosa replied: "Edward Zuma is a piece of vomit and he is talking rubbish."

In Wednesday's letter, Zuma said he was yet to hear Phosa speak out on the banks that are currently facing a probe by the Competition Commission.

"Again, I remembered that you can't speak against them as they buy you bread and milk."