Pakistan's coalition government appears close to collapse after former premier Nawaz Sharif threatened to quit if judges sacked by ousted president Pervez Musharraf are not restored.
Mr Sharif's threat comes a day before the leaders of the coalition, headed by the party of slain ex-prime minister Benazir Bhutto, are to hold crucial talks on the issue.
But in a sign that Bhutto's party is seeking to move forward on its own, MPs have backed her widower, Asif Ali Zardari, to replace Mr Musharraf as president of the nuclear-armed Islamic republic.
"If the judges are not restored we will perhaps be forced to sit in the opposition," Sharif says.
"We will not try to bring the government down. But, of course, then we have no choice but to sit in the opposition."
Musharraf stepped down on Monday in the face of coalition threats to impeach him over constitutional violations, including his ousting of dozens of senior judges in order to push through his re-election in November.
Western countries who count on Pakistan as a frontline ally in the US-led "war on terror" have called for stability in the wake of his departure.
Meanwhile, at least 12 people have been killed in twin suicide bombings outside a munitions factory in the Pakistani city of Wah, police say.
They say the bombs went off as workers left the factory.
Correspondents say the city is usually under heavy security, as it is home to a large industrial complex producing conventional arms and ammunition.