ISLAMABAD -- Pakistan's Supreme Court nullified on Wednesday a controversial deal that had given President Asif Ali Zardari and thousands of other government officials amnesty from prosecution on corruption charges, a decision likely to further weaken Zardari's shaky hold on power.

The ruling could open the door to additional legal challenges against Zardari. Although he still has immunity from prosecution under the constitution, opponents plan to contest that by arguing that Zardari is technically ineligible for the presidency.