The widely welcomed amendments to the Protection of State Information Bill were unanimously approved on Monday by the members of the ad hoc committee finalising the draft bill.

After nearly three years of controversy and drafting changes, the bill will come another step closer to law on Tuesday when MPs on the committee vote on the final draft of the bill, to which these changes have been added.

"Today we approved the amendments. We now have to approve the amended bill, with the changes made by the NCOP [National Council of Provinces] incorporated," committee chairperson Cecil Burgess said.

The bill will then be sent back to the National Assembly – which approved an earlier, considerably different version in late 2011 – where it will be put to the vote in its new form.

Burgess suggested this could happen as early as Thursday.

Opposition politicians and civil rights groups who fought tooth and nail against earlier drafts of the legislation, deemed a throwback to apartheid-era state secrecy, have welcomed the changes brought by the NCOP.

These include a clause offering protection from prosecution for anybody who reveals classified information to bring to light a crime.

The NCOP members also dropped from the bill a clause which sought to make the new law trump the progressive Promotion of Access to Information Act, and another which imposed heavy jail sentences for revealing any information relating to a "state security matter".

Critics, including Cosatu, argued persuasively the latter clause would create an all-powerful intelligence apparatus.

On Monday the Democratic Alliance and the African Christian Democratic Party signalled that despite approving the amendments, their overall opposition to the bill stands.

DA MP Dene Smuts said her party believed the bill remained unconstitutional because it sought, wrongly, to govern keeping records by provincial archives.

"Public-record keeping is a provincial competency and Parliament ought not to be legislating on it at all," she said.

Smuts said an alternative and separate argument was that, if the provision on archives were to stand, the bill should not have been processed as a section 75 but a section 76 bill, in terms of the Constitution, because it dealt with provincial powers.

If the bill is passed, the DA is expected to appeal to President Jacob Zuma to send it to the Constitutional Court for review before he signs it into law. Failing this, the opposition seems set to challenge the legislation in court.