South Africa's ex-intelligence minister has warned the governing African National Congress (ANC) against pushing a secrecy bill through parliament.
The Protection of Information Bill would allow the government to classify material that is currently not secret.
In an open letter, Ronnie Kasrils said it could have unintended consequences and silence whistle-blowers.
"It will certainly undermine public trust in the intelligence and security services," he said.
The BBC's Karen Allen in Johannesburg says the proposed legislation has been met with a torrent of criticism from across the political divide.
'Freedom of expression sacrosanct'
Mr Kasrils is the latest high-profile figure to speak out against the controversial plans.
He said the existing apartheid-era legislation needed reform, but said a public interest defence needed to be included to protect whistle-blowers.
"Since freedom of expression is a dearly won principle of our liberation struggle, it needs to be treated as sacrosanct outside the narrow national-security sphere," he said.
Mr Kasrils initially introduced the Protection of Information Bill in 2008 when he served under former President Thabo Mbeki.
The bill was withdrawn under huge public opposition, according to South Africa's Times newspaper.
Current State Security Minister Siyabonga Cwele introduced a revised bill last year.
The close relationship between senior politicians and the intelligence services has triggered alarm bells in South Africa in recent years, our correspondent says.
Observers have accused the ANC of flexing its muscles on the bill, now that local government elections have been held.
On Wednesday, trade unions threatened to sue the government in the Constitutional Court if the bill was pushed through in its current form.
One of the Congress of South African Trade Unions's biggest objections was the lengthy prison sentence someone would face if they came into possession of a classified document and passed it on to anybody but the police, the Sapa news agency reports.