"The government will hold its last cabinet meeting on the 20th of this month ... and then will come the change," Hariri told journalists on Thursday after a cabinet meeting.
His announcement came amid mounting tensions between Lebanon's political masters in Damascus and the United States.
Hariri, a business tycoon who has been a mainstay of Lebanese politics for years, declined to say whether he would head the new government, saying only that it was necessary to await the results of discussions.
He is the political arch-rival of pro-Damascus President Emile Lahud, whose mandate was extended last week after the Lebanese parliament bowed to Syrian pressure to change the constitution to allow the move.
Lahud's mandate was extended
last week under Syrian pressure
The vote, which has already seen four cabinet ministers resign in protest, came a day after the UN Security Council adopted a resolution backing Lebanon's sovereignty and implicitly censuring Damascus.
The resolution, which also called for foreign troops to withdraw from Lebanon and for all militia to be disbanded, was drafted by the United States and France, traditionally a friend to Syria.
Damascus maintains at least 16,000 troops in Lebanon, a holdover from a larger contingent sent in during the 1975-1990 civil war, and supports Hizb Allah, which controls areas in the south of the country.
The amendment giving Lahoud another three years in office came under fire both from abroad and from Christians in Lebanon who are the most vocally opposed to the role of Damascus.
Hariri (R) quit once in 1998 when
Lahud (L) was elected president
Hariri has been prime minister since 1992, apart from a brief period in the political wilderness from 1998 to 2000. The latest government was formed in April 2003.
Hariri's resignation in 1998 came about the first time former army head Lahud was elected president with overwhelming support from Damascus. Analysts said Hariri was worried about a potential weakening of his position by a strong new president.
He was first asked to form a government as Lebanon was emerging from the massive destruction caused by years of civil strife.
Born in 1944, Hariri made billions of dollars running a construction company in Saudi Arabia and then during Beirut's reconstruction
Admirers of Hariri see him as responsible for getting the country's economy back on its feet after the civil war, while his critics accuse him of being a spendthrift in charge of a corrupt administration.