Pakistan's Supreme Court on Friday struck down a new law exempting members of the government from being tried for contempt, clearing the way for legal proceedings against the prime minister.
Parliament passed the bill last month after the Supreme Court dismissed Yousuf Raza Gilani as prime minister and convicted him of contempt for refusing to reopen multi-million-dollar corruption cases against President Asif Ali Zardari.
But on Friday, a five-member bench of the Supreme Court headed by chief justice Iftikhar Muhammad Chaudhry declared the law "unconstitutional".
New Prime Minister Raja Pervez Ashraf now has until August 8 to indicate whether he will follow a court order to write to authorities in Switzerland, asking them to reopen the cases against Zardari.
Last month, the Supreme Court made veiled threats Ashraf could suffer the same fate as Gilani if he refuses to do so.
For more than two years, the government has resisted judges' demands to reopen investigations into Zardari, arguing he enjoys immunity as head of state.
The showdown could force elections before February 2013 when the government would become the first in Pakistan's history to complete an elected, full five-year mandate.
The allegations against Zardari date back to the 1990s, when he and his wife, late premier Benazir Bhutto, are suspected of using Swiss bank accounts to launder $12 million allegedly paid in bribes by companies seeking customs inspection contracts.
The Swiss shelved the cases in 2008 when Zardari became president and the government insists the president has full immunity as head of state.
But in 2009, the Supreme Court overturned a political amnesty that had frozen investigations into the president and other politicians, ordering that the cases be reopened.
Zardari had already signed the contempt law, which sought to exempt government figures, including the president, prime minister and cabinet ministers.