Ambassador Husain Haqqani, now in Islamabad for consultations, is alleged to have arranged a Zardari memo to be sent to Adm. Mike Mullen, former U.S. Joint Chiefs of Staff chairman, seeking American help to prevent a Pakistani military coup after the May killing of al-Qaida leader Osama bin Laden in Pakistan.
Haqqani, who has denied the allegations and offered to resign, met Zardari Sunday and planned to meet with him again Monday, an aide to the ambassador told The New York Times.
The unsigned memo allegedly offered help with the war on Islamic militants and other issues in return for U.S. assistance, the report said.
Former Prime Minister Nawaz Sharif, now the main opposition leader, joined others in demanding an investigation, saying he would go to the Supreme Court if the Zardari government does not establish the authenticity of the memo in two days.
The Financial Times reported that Mansoor Ijaz, a Pakistani-American businessmen, wrote a newspaper article about the alleged memo in October but his claim was dismissed by the Pakistani government as a "fantasy."
The issue resurfaced after Mullen reportedly said he had received a memo but did not find it credible, the Financial Times said.