Lebanese Prime Minister Rafiq Hariri and his cabinet have resigned after weeks of political uncertainty.
Hariri has served as PM for 10 of the last 12 years
He submitted the resignation to President Emile Lahoud during a brief meeting at the presidential palace.
Mr Hariri - a billionaire businessman - has led Lebanon for most of the period since the civil war ended in 1990.
Government has been paralysed over a dispute between the president's Syrian political backers and opponents of Syria's domination of Lebanon.
Mr Hariri - who has close ties with Syria, but is a political rival of the president - has said he will not head a new government.
Lebanon has been in political deadlock since parliament approved a controversial constitutional amendment to extend the president's mandate as a direct result of Syrian pressure.
Mr Hariri initially opposed the three-year extension, but eventually came into line after Syria put pressure on him.
Four cabinet ministers quit the government in September over the constitutional change, which has also split the country into pro- and anti-Syria camps.
Mr Lahoud is expected to meet the main parliamentary
blocs on Thursday to start the process of finding a replacement
A statement by Mr Hariri said internal political differences were behind his decision, but correspondents say it is too early to say whether it spells the end of his political
"Facing challenges, any challenges, can only be done through a unified domestic front that meets the goals of the Lebanese people... but these objectives have been confronted by known
political realities," Mr Hariri said.
The political crisis has coincided with a standoff with the United Nations, which has called for foreign - ie Syrian - forces to withdraw from Lebanon and an end to foreign interference in Lebanese elections.
Damascus has so far resisted UN pressure and Beirut maintains that the approximately 14,000 Syrian troops in Lebanon are there to guarantee stability at the government's request.
Mr Hariri has been the driving force behind the reconstruction of Lebanon after the civil war which raged here until 1990.
He has close relations with the Saudi royal family and the French president, Jacques Chirac.
Correspondents say his departure has cast a shadow over the economic health of Lebanon which is $32bn in debt.
It also means a much less seasoned team will face the international pressure over Syria's role in Lebanese affairs.