African National Congress (ANC) MPs on Friday called for an extension of the looming deadline to complete drafting the Protection of Information Bill after ruling party ally Cosatu voiced strong opposition to the draft law.

"We want to request an extension of the the life of this ad hoc committee," ANC lawmaker Luwellyn Landers said after a meeting of the party's study group on the legislation.

"The deadline of June 24 is inadequate."

Landers suggested that the new deadline be somewhere in the second week of the new parliamentary session, which starts on July 22.

Chairperson Cecil Burgess then agreed to seek an extension from the Speaker's office.

This move marks a turnaround for the ANC majority on the committee, which had in the past two weeks seemed determined to drive the bill through Parliament despite widespread calls that it was unconstitutional.

Burgess had stated that he would not entertain endless debate and instead opened formal clause by clause deliberations and put contentious points to the vote, leaving the opposition outnumbered in its call to narrow the scope of the so-called "secrecy bill".

On Tuesday, Cosatu threw down the gauntlet on the bill.

It asked for talks with the ANC and threatened to launch a Constitutional Court challenge should the bill be passed in its current form.

The trade union federation said the provisions of the bill could be abused to cover up corruption and the lengthy prison sentences prescribed for people who make classified information public posed a threat to whistle-blowers.

A day later the Treatment Action Campaign weighed in, saying the bill would make it impossible for civic organisations to hold government to account.

The bill has long been seen as a threat to media freedom but rights campaigners say the public as a whole would be affected if all organs of state are allowed to classify information.

The Right 2 Know Campaign, born out of opposition to the bill, said the legislation would rob poor communities of access to information on service delivery.