Gebran Bassil on Sunday accused Prime Minister-designate Saad Hariri of sidelining President Michel Aoun, the country’s highest Christian official, in choosing his future ministers.
The process stalled since the appointment two months ago of Mr Hariri to the premiership for the fourth time, as sectarian leaders disagree on their ministerial share of the next Cabinet.
"Why is Saad Hariri assuming that he can nominate Christian ministers on behalf of the resident and Christians? We are not second-class citizens," Mr Bassil said in a televised speech.
An MP and former minister, Mr Bassil, 50, leads the Free Patriotic Movement, founded by his father-in-law, Mr Aoun.
It is Hezbollah’s biggest Christian ally and the largest party in parliament.
Mr Bassil's speech followed one this week by Hezbollah leader Hassan Nasrallah and a TV interview Druze leader Walid Joumblatt.
Sectarian leaders have evaded responsibility for the stalled government formation, choosing instead to embroil themselves in disputes as the country plunges further into crisis.
But a new, clean government capable of enacting reforms is necessary for Lebanon to gain access to debt relief and international loans.
“The prime minister-designate does not consult us because we did not endorse him and he says he does not want to anger his allies,” said Mr Bassil, insisting that the FPM did not want to play a role in the next government.
"But he consults Hezbollah and expects it to send names for future ministers."
Hezbollah, the FPM and their Christian rivals the Lebanese Forces, did not endorse Mr Hariri’s bid for the premiership.
But the militant group and its Shiite ally – the Amal Movement – are insisting on nominating Lebanon’s future finance minister.
In Lebanon’s power-sharing system, the president must be a Maronite Christian, the prime minister a Sunni and the Speaker of parliament a Shiite.
Ministerial portfolios are not assigned to specific sects.
The quotas were meant to promote shared decision-making after 15 years of civil war that ended in 1990, but the system has been accused by pro-reform activists of promoting nepotism and corruption.
Lebanon has been run by a caretaker government since August, when Hassan Diab resigned as prime minister after a deadly blast at Beirut port, compounding an economic crisis.
Mr Hariri succeeded him a year after a mass protest movement against corruption, sectarian politics and foreign meddling forced him to resign.
Mr Bassil was criticised by Lebanese demonstrators, who took to the streets last year to demand reform.
They regard him as a sectarian leader with ambitions for the presidency, an ambition now compromised by US sanctions imposed on him for corruption in November.
Mr Bassil's opponents accuse him of having forged an alliance with Hezbollah and of warming to the Syrian regime to succeed his father-in-law as president.
He has refused to give up his alliance with the militant group although he said the FPM and Hezbollah would launch a dialogue to reassess their partnership.
He has championed “Christian rights", which he says are under threat from Syrian refugees, to whom he refers as “migrants” and “foreigners”.