The bill carries 25-year jail terms despite an outcry from activists and editors who argue it will muzzle investigative journalism, with protests planned in several cities.

“The Protection of State Information is not only flawed, it is insulting to all South Africans to be asked to stomach legislation that could be used to outlaw whistle-blowing and investigative journalism,” said Tutu.

The bill would punish the possession or release of classified documents, even if the information serves society’s best interest, making the “state answerable only to the state”, he said.

Several protests have been planned for Tuesday with the National Press Club calling on people to wear black to show their opposition. They’ve dubbed the day “Black Tuesday” in a nod to a 1977 apartheid press crackdown.

Tutu urged lawmakers to reject the bill, which will replace an apartheid-era law.

“This is not to suggest that those presently holding the reins of power intend to use the legislation to muzzle anyone or to cover anything up,” he said.

“But, equally, there are presently a sufficient number of investigations (to the credit of government) into alleged corruption by members of this and previous governments to warrant treading with extra care.”

But the African National Congress which holds the majority in parliament has refused to bow to pressure and says the bill stands up to international best standards.

“This government will never compromise the security of its people for the sake of one scoop of a journalist,” ANC caucus spokesman Moloto Mothapo said on Talk Radio 702.

The Nelson Mandela Centre of Memory also added its concerns, saying the bill fails to strike a balance between free speech and protecting legitimate state secrets.

“Much has been achieved, but the bill is not yet at a point where it can be said to have met the above-mentioned standards and aspirations,” it said.

South Africa’s press regularly uncovers allegations of graft that reach to the highest level of government. Newspapers warned in identical front page editorials that Tuesday will end “as a day of triumph or of shame for our democracy”.

Lawmakers who vote for the bill “take personal responsibility for the first piece of legislation since the end of apartheid that dismantles an aspect of our democracy — a betrayal that will haunt them forever,” it said.

The bill was first drafted in 2008 and reworked this year in marathon sessions by a special committee which narrowed far-sweeping classification powers, but the ANC refused to budge on other chief concerns.

“Unfortunately, however, while the changes are necessary and valuable, they are not sufficient to render the bill safe for democracy,” said the South African National Editors Forum in a letter to lawmakers urging them to reject the bill.

“On the contrary, in its current form, the bill represents an attack on principles of open democracy that are deeply embedded in our constitution and our national life.”

The public interest defence was “crucial to ensuring that the bill does not become an instrument to suppress information that may reveal serious wrong-doing”, it said.

After the vote Tuesday, the bill still needs approval from the lower house of parliament before it heads to President Jacob Zuma’s desk to be signed into law. Press advocacy groups have already threatened a challenge in the courts if it becomes law.