This file photograph taken on August 18, 2020, shows a sign in front of the building of the UN-backed Special Tribunal for Lebanon (STL) at Leidschendam, before the expected verdict on the 2005 murder of former Lebanese premier Rafiq Hariri. AFP
Appeals judges at the Netherlands-based court said trial judges "erred" in 2020 by acquitting the two men, in finding that there was a lack of evidence.
Set up in 2009 to try those responsible for the huge explosion in downtown Beirut that killed Hariri and 21 others, the court convicted Salim Ayyash, a member of the Lebanese Shiite Muslim movement Hezbollah.
But they acquitted Hassan Habib Merhi, Hussein Oneissi and Assad Sabra, saying there was not enough evidence to prove their involvement.
Appeals judges however granted an appeal by prosecutors, who asked that the acquittals be overturned for Merhi and Oneissi.
"The appeals chamber has unanimously decided to reverse the acquittals of misters Merhi and Oneissi. We unanimously find Mr Merhi and Oneissi guilty," presiding judge Ivana Hrdlickova said.
"The appeals chamber will issue arrest warrants for them later this afternoon," she added.
All four men were tried in absentia over the February 2005 attack, when a suicide bomber detonated a van full of explosives as Hariri's armoured convoy passed on the Beirut waterfront.
'Beyond reasonable doubt'
The case against all four men relied almost exclusively on circumstantial evidence in the form of mobile phone records that prosecutors said showed a Hezbollah cell plotting the attack.
The appeals judges agreed with prosecutors that evidence showed that mobile phones used by Merhi and Oneissi, together with a third mobile, was proof of their involvement in the attack on Hariri.
"The evidence shows a significant number of exchanges between the (third) mobile phone and both Misters Merhi and Oneissi in the hours following the assassination of Mr Hariri," Judge Hrdlickova said.
"Telecommunication evidence shows that Misters Merhi and Oneissi... simultaneously discarded their phones after the attack," she added.
It had been established "beyond reasonable doubt" that Merhi "knowingly and willingly entered into an agreement to participate in the commission of a terrorist act, namely the assassination of Mr Hariri by means of an explosive device," the judge said.
Moreover, Merhi could have foreseen that in attacking Hariri in such a public place "other people could have died".
Similarly, Oneissi too "knowingly and willingly" agreed to participate "in the commission of a terrorist act", said Hrdlickova.
Hariri's assassination sparked a groundswell of protest at the time, seeing Syrian troops quit Lebanon in April 2005 after a 29-year deployment which peaked at 40,000 troops.
Hassan Nasrallah, the head of the Hezbollah movement, has refused to hand over any of the suspects or to recognise the UN-backed court, which has issued an international warrant for the arrest of Ayyash.
The court said in April last year that Ayyash cannot appeal against the verdict until he turns himself in.
The appeals judges are now to hand down sentences for Merhi and Oneissi, with Ayyash already given five concurrent jail sentences.
The Lebanon tribunal is expected to close after the appeals phase because of a cash shortage, with a further case against Ayyash over attacks on several politicians likely to go unheard.