Public Enterprises Minister Pravin Gordhan also told Reuters on Sunday the alleged incident "will be thoroughly investigated" and anyone responsible charged.
Without giving any details, Gordhan said an intense battle was taking place "between those who want South Africa to work and thrive and those who want to corruptly enrich themselves".
Faced with political pressure, De Ruyter resigned on 14 December 2022 after failing to solve a crisis in Eskom that has led to record power cuts in Africa's most industrialised economy.
After officially taking office in January 2020, De Ruyter led a company-wide clampdown on corruption and organised criminal behaviour, including sabotage of infrastructure, at Eskom plants. His last day in office will be 31 March 2023.
"Eskom cannot comment further on the poisoning incident involving the group chief executive, which occurred during December 2022, as the matter is subject to police investigation," the utility's head of security said in a statement.
Reuters could not immediately reach De Ruyter for comment.
The alleged cyanide poisoning was first reported by specialist energy publication EE Business Intelligence on Saturday.
Opposition party the Democratic Alliance on Sunday called for decisive action against criminal syndicates that it said were "hell-bent on cementing their stranglehold on Eskom that is destroying the economy".
The South African police services did not immediately respond to Reuters' request for comment. Eskom's board chairman Mpho Makwana was also unavailable.
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