The sale is due in January in New York by Guernsey's auction house. The seller is Christo Brand, Mandela's former prison guard in the notorious jail.
But Nathi Mthethwa, South Africa's culture minister, said there had been no consultation with the government.
"This key belongs to the people of South Africa," he said.
"It is not anyone's personal belonging," the minister added.
Guernsey's says its planned auction on 28 January is to raise funds for a memorial garden and museum around Mandela's burial site.