Lebanon's Prime Minister Saad al-Hariri has replaced his chief of staff after his Future Party lost more than a third of its seats in parliamentary elections a week ago.

The Future Party won 20 seats, down from the 33 won in the country's last elections held in 2009.

In a post-election speech last week Hariri said the party had been expecting a better result and there had been "gaps" in how it conducted its campaign, for which people would be held responsible.

Hariri's office announced the resignation of Nader al-Hariri, a cousin of the prime minister, late on Saturday. It said Mohamed Mnaimne had replaced him in a temporary capacity.

Despite Future's losses, Hariri is still the frontrunner to form the next government, as the Sunni Muslim leader with the biggest bloc in parliament. Lebanon's prime minister has to be a Sunni under its sectarian power sharing system.

The Iran-backed Hezbollah group and factions and individuals that support its possession of weapons made significant gains last Sunday, winning more than half the seats in parliament.

Hezbollah's powerful arsenal has been a point of contention in Lebanon for years.The staunchly anti-Hezbollah Lebanese Forces won 15 seats, almost doubling their MPs.

The current parliament's term expires May 20, and difficult negotiations are expected over the division of positions in the new government.