Barnaby Joyce said at a press conference in Armidale, his rural New South Wales seat, that he would step down as Nationals leader and deputy prime minister at a party meeting on Monday.
Australia's embattled deputy prime minister, under pressure over an extramarital affair, faced a call to step down as leader of his party, the first such call from a member of the party, which is part of the ruling coalition.
A state branch of the Australian National Party has pulled its support for Deputy Prime Minister Barnaby Joyce because of reports about an extramarital affair with his former press secretary, the party's Western Australia leader said.
Barnaby Joyce leads the rural-based National Party, the junior partner in the centre-right government led by Malcolm Turnbull's Liberal Party, a political alliance that has existed since 1923.
Malcolm Turnbull, whose coalition holds a razor-thin majority of just one seat, said on Thursday that Barnaby Joyce had shown a "shocking error of judgment" for conducting an affair.
Australian Prime Minister Malcolm Turnbull on Saturday appointed his foreign minister as acting prime minister as he tried to stem the fallout of a citizenship crisis that has cost his government its parliamentary majority.
Australia's High Court ruled on Friday that Deputy Prime Minister Barnaby Joyce is ineligible to remain in parliament, a stunning decision that cost the government its one-seat parliamentary majority and forced a by-election.
The Australian government accused a New Zealand opposition party of conniving to bring it down by revealing that Australia's deputy prime minister is a New Zealand citizen, and thus ineligible to sit in parliament.