President Musharraf has said he would rather resign than be impeached
Pakistan's ruling coalition parties say they have agreed "in principle" to start impeachment proceedings against President Pervez Musharraf.
Party representatives are said to be looking at a draft impeachment resolution, but further details about how it may proceed were not available.
The president's allies were defeated in elections in February, but he has so far resisted pressure to quit.
Latest reports say Mr Musharraf has cancelled a trip to the Olympics.
News agencies quoted the foreign ministry as saying Prime Minister Yousef Raza Gilani would attend the Games' opening ceremony in Beijing instead.
STEPS TO IMPEACHMENT
Impeachment proposers need 50% majority in Senate or National Assembly
President given notice of impeachment, and has three days to respond
Joint session of Senate and Assembly must be held between 7 and 14 days later to investigate charges
If resolution presented, joint session must approve with two-thirds majority
OTHER POSSIBLE SCENARIO
President dissolves parliament, placing country under emergency rule to prevent impeachment
But there has been no confirmation from the president's office.
Correspondents say the question of whether or not to impeach Mr Musharraf has threatened to divide the coalition.
Asif Ali Zardari's Pakistan People's Party (PPP) formed an alliance with the PML-N of former Prime Minister Narwaz Sharif after defeating Mr Musharraf's political allies in February.
But they have since been split over the issues of presidential impeachment and the reinstatement of judges sacked by President Musharraf during a state of emergency in November.
Exactly what course of action Mr Sharif and Mr Zardari have decided to take is expected to be announced at a joint news conference later.
The BBC's Mark Dummett in Islamabad says an impeachment would take Pakistani politics into new territory since no Pakistani leader has faced impeachment before.
There has been no response from the president himself, but his supporters in parliament have said they will fight any attempt to remove him.
The PPP and PML-N coalition parties do not have the two-thirds majority needed to pass an impeachment resolution, so would need opposition support.
Correspondents say some opposition parties do not support impeachment and nor do some members of the PPP itself - so there is scepticism that such a majority could be reached.
The president has previously said he would prefer to resign than face impeachment.
Last year, he gave up control of the army, the country's most powerful institution, but he retains the power to dissolve parliament.
How the military reacts to any efforts to oust him would be crucial in determining his fate.