"But the supreme court could say that [the NRO] was parliament's decision in the first place and ... [it could] go back to parliament to decide whether the deal should be ratified.

"The third [possible decision] is to declare the NRO illegal. If that happens, it would call into question the political office of Zardari and a number of other officials."

Amnesty agreement

The deal, which was announced two years ago by then-president Pervez Musharraf, allowed Zardari and Benazir Bhutto, his wife and a former prime minister, to return from self-exile without facing corruption charges.

But the NRO has been branded unconstitutional and the debate over its legitimacy has received a great deal of coverage in Pakistani media.

The amnesty covers more than 8,000 officials and civil rights activists argue that it was unjust to help so many politicians escape prosecution for alleged wrongdoing.

As president, Zardari, who has denied a slew of corruption claims against him, enjoys general immunity from prosecution.

But the supreme court could choose to challenge his eligibility for the post if it decided that the amnesty is illegal.

Legal and political analysts are divided on whether this is likely, and most expect the process to take several months to run its course.

The debate comes as Pakistan's army battles Taliban fighters in its tribal regions.

The potential for a crisis to erupt from the supreme court ruling is a cause for concern in the US, whose administration wants Pakistan to remain focused on its campaign against the Taliban.