President Jacob Zuma on Thursday sent the Protection of State Information Bill back to Parliament, saying he could not sign it into law because it was incoherently drafted and therefore unconstitutional.

"It is my opinion that the bill would not pass constitutional muster," Zuma said in announcement that was widely welcomed by critics who have campaigned against the legislation for years.

The president singled out two sections of the bill as problematic but his office indicated that a letter sent to Speaker Max Sisulu mandates lawmakers to revise the contentious official secrets bill as a whole.

"I have referred the bill back to the National Assembly for reconsideration, in so far as sections 42 and 45 lack meaning and coherence, and consequently are irrational and accordingly are unconstitutional," Zuma said to parliamentary reporters.

Section 45 criminalises the improper classification of state information and provides for prison sentences of five to 15 years, depending on the level of wrongful classification. It notably makes it a crime to classify information to conceal corruption or influence a tender process.

Ironically, advocacy groups have welcomed this section for offering those who reveal classified information to expose state wrongdoing protection from prosecution.

Section 42 purports to deal with failure to report possession of a classified document but refers back to an earlier section that sets out the maximum classification period, as stipulated in the National Archives Act.

Pressed by the media, Zuma declined to explain in full why he found these parts of the bill problematic, prompting hours of speculation as to the exact provisions in question and the extent of possible amendments.

Confusion grew when his spokesman Mac Maharaj indicated that Zuma may have meant to refer to different sections of the bill but he later retracted this.

Shortly after Zuma announced his decision, Sisulu informed the National Assembly that an ad hoc committee would be established to deal with the president's reservations about the bill.

Rising to address the Assembly, State Security Minister Siyabonga Cwele welcomed the president's decision, because it would strengthen the legislation.

Since the bill was adopted in April, Zuma has been lobbied not to sign it into law, but to refer it to the Constitutional Court for certification.

Asked why he did not take this route, the president responded that the Constitutional Court was not in a position to amend the bill, and that only lawmakers were.

"I sent it to the people who will fix what I think is wrong."

The bill was deemed a throwback to apartheid-era state secrecy when it was first introduced, and over almost three years of drafting the ANC was forced to retreat from some of its most controversial clauses.

But critics - among them ANC alliance partners and veteran human rights lawyer George Bizos -- have insisted that it remained unconstitutional.

Their objections revolve around more provisions than those Zuma singled out, including the fact that the bill places a low burden of proof on the state for crimes such as espionage.

Last month, the Nelson Mandela Centre for Memory called for the bill to be withdrawn and rewritten from scratch because, it said, it would create a regime of dealing with state information that ran parallel to that envisaged in the Constitution and the progressive Promotion of Access to Information Act (PAIA).

"This was the basis for the secrecy and lack of accountability which characterised the apartheid system," it said.

The Right2Know campaign welcomed Zuma's decision and said he had mentioned "only two of many draconian aspects of the bill" and urged a comprehensive redraft.

"We call on members of Parliament to seize this opportunity to redeem themselves and redraft the bill to bring it in line with the values of openness and transparency," the group's national co-ordinator Mark Weinberg said.