Saad Hariri had stepped down as PM-designate on Thursday after nine months of futile deliberations, citing deep disagreements with President Michel Aoun.
Hariri's exit came amid a financial collapse branded by the World Bank as one of the planet's worst since the 19th century.
His departure left the country rudderless as Lebanon faces soaring poverty, a plummeting currency, angry protests and shortages of basic items from medicine to fuel.
Hariri had been nominated premier-designate in October 2020 to replace Mustafa Adib, a relatively unknown diplomat.
Adib had been nominated just weeks after the Beirut port explosion, but quit less than a month later over resistance to his proposed line up.
Outgoing Prime Minister Hassan Diab, who resigned in the wake of the August 4 explosion, has stayed on in a caretaker capacity until political leaders can agree on a new premier.