The ANC used its majority strength in a parliamentary committee on Friday to adopt a final draft of the Protection of Information Bill, which critics immediately vowed to challenge in court.

"We think it will prove unconstitutional," said Alison Tilley of the Open Democracy Advice Centre after the bill was finalised.

At issue is the ruling party's refusal to include a public interest defence to protect people who disclose classified information to expose government wrongdoing from prosecution, and potentially imprisonment.

The Democratic Alliance, Inkatha Freedom Party and African Christian Democratic Party all moved amendments to allow such a defence, but were outvoted eight to four by the ANC.

"We think that the bill is constitutional," said ruling party MP Luwellyn Landers after the process was completed a year after the bill was introduced.

The state security ministry also welcomed the final draft.

The ACDP's Steve Swart said he would petition President Jacob Zuma to refer the legislation to the Constitutional Court before he signs it into law.

"We have made very good progress on the bill, apart from the public interest defence. It is a major issue and it makes the bill open to challenge and therefore we will ask the president to refer it to the Constitutional Court for its decision."

Tilley also indicated it was unlikely a challenge would wait until somebody was charged under the new legislation.

"We will be able to move sooner rather than later. Our position is that the point where the president certifies the bill as constitutional, is the point at which it can be taken on review.

"We are deeply disappointed that a public interest defence was not included," she added.

"It means that journalists and others are not protected for disclosing evidence of state malpractice."

Both the opposition and civil society critics welcomed significant concessions made by the ruling party on other issues, after the ANC came under pressure from rights groups and its alliance partner Cosatu.

The ANC agreed to narrow the definition of national security in the bill and to limit the power to classify information to the intelligence and security services. Other departments would have to seek special permission to keep secret files.

It also agreed to remove provisions that prescribed minimum sentences for all acts criminalised in the bill, apart from espionage.

The opposition said these changes were victories in their struggle to rewrite the initial draft, which critics termed an attempt to return to apartheid-era secrecy.

Said the DA's Dene Smuts: "We have rewritten this bill as a decent piece of intelligence and classification law.

"It is only in the offences clauses that it has gone wrong, to our enormous regret.

"We tried hard to get a public interest defence for our media and anybody else. And we tried hard to get rid of the possession clause."

Clause 15 of the bill states that anybody who knowingly comes into possession of a classified document must report this to the police and return the document to either the police or the state intelligence agency.

This has been described by the DA as an untenable attempt to force journalists to betray sources of leaked documents to the authorities.

The party also criticised a clause that introduces maximum prison sentences of 25 years for disclosing a classified document containing information about the intelligence services -- five times the top sentence for disclosing other classified information.

The state security ministry, which kept a watchful presence in the committee throughout, reiterated its position that a public interest defence would render the bill worthless.

"We reiterate our assertion -- which we made in response to the public hearings -- that to include such a defence would be to shred the bill even before it becomes law."

It also argued the bill provided adequate protection for whistleblowers, a point Cosatu still contests.

The bill is likely to be put to the vote in the National Assembly before September 15.

The ad hoc committee will meet one last time on Monday to vote on the final draft to include a cost estimate. This has become necessary since the redraft provides for the establishment of a classification review panel, but is now considered a mere formality.