Embattled Prime Minister Yousuf Raza Gilani will appear before Pakistan's Supreme Court to face contempt of court
proceedings that could see him convicted and disqualified from office.
It is only the second time that the court has initiated contempt proceedings against a sitting prime minister, plunging Gilani's weak government deeper into a crisis that could force early elections within months.
Pakistan's highest court summoned Gilani to explain his refusal to ask Switzerland to re-open graft cases against President Asif Ali Zardari in what some have interpreted as a step towards a constitutional coup.
The government is also under immense pressure from the army and judiciary over a memo asking the Americans to prevent a feared coup last May.
Gilani is expected at court on Thursday under tight security in the heavily protected constitutional sector of Islamabad from 04:00 GMT.
Officials have been tight-lipped on what the prime minister and his lawyer will say, but commentators doubt that Gilani will decide to risk six months in jail and being stripped of office.
His lawyer, Aitzaz Ahsan, has said Gilani may as well do what the court wants and write to Switzerland because Zardari has immunity from prosecution as long as he remains in office.
Ahsan is a senior leader in Zardari's Pakistan People's Party (PPP) and well respected by the judiciary for his role at the vanguard of a lawyers' movement that forced the government to reinstate independent judges in March 2009.
Supreme Court judges have six options on how to proceed which include finding Gilani in contempt, disqualifying the prime minister and president, and holding early elections.
Analysts say Gilani has to either resign or find a way of satisfying the court order if he wants to keep his job.
Some have suggested that the PPP could sacrifice Gilani to protect the president, who has been accused over the controversial May 10 memo that sought US assistance to curb the powerful military.
His ambassador to Washington, Husain Haqqani, was forced to resign and a judicial inquiry is examining who was responsible for the note.
The domestic turmoil has temporarily overshadowed a drastic deterioration in relations with the United States that plummeted disastrously in 2011 over a series of crises, largely America's covert killing of Osama bin Laden on May 2.