Fouad Siniora, the prime minister, said on Monday: "We tell the criminals that we will not give up our right, no matter what the difficulties and obstacles are.
"Our aim is to achieve justice and only justice. Without it - and without knowing the truth - the Lebanese will not rest and we cannot protect our democratic system and political freedom now or in the future."
Siniora, whose anti-Syrian majority dominates the Lebanese cabinet, convened the session despite objections by Emile Lahoud, the president, and the resignation of six Shia ministers.
The resignations left the 18 remaining ministers approving the United Nations document, and they defended the cabinet's decision as legal.
Ghazi Aridi, the information minister, quoted Siniora as saying: "It is 100 per cent constitutional."
A UN investigation has implicated top Syrian officials, but Syria has denied any role in the assassination.
Rafiq al-Hariri had widespread
appeal in Lebanon
Al-Hariri was killed along with 22 others in a truck bombing in February 2005.
The assassination caused large anti-Syrian protests in Beirut, the Lebanese capital, and led to the withdrawal of Syrian troops from Lebanon, ending a 29-year military presence.
Subsequent elections brought an anti-Syrian majority to power in parliament and the Cabinet.