Former South African President Thabo Mbeki has hailed a recent constitutional court ruling against incumbent President Jacob Zuma on his home upgrade case.

The court ruled on March 31 that Zuma violated the constitution by ignoring the public protector's request for him to repay part of the state money lavished on security upgrades at his private home in Nkandla.

In a 4,000-word article made public on Monday, Mbeki called the judgment a "critical contribution" to the evolution of South Africa's democracy.

The failure to observe the fundamental values of South Africa's constitution threatens the survival of the country's democracy, Mbeki said.

The ruling has prompted calls from outside and within the ruling African National Congress for Zuma to resign.

"This decision has evoked much understandable and inevitable political discussion and activity in our country, which has included calls for the immediate removal of the president of the republic," Mbeki said in the article.

Zuma has said he would abide by the ruling and repay the money.