By Colin Packham and Tom Westbrook
CANBERRA/SYDNEY (Reuters) - Australia's widening citizenship crisis entangled a seventh politician on Friday, a key independent senator whose support is critical for Prime Minister Malcolm Turnbull to pass legislation through a hostile Senate.
Senator Nick Xenophon said he may hold dual citizenship, Australian and British, which would make him ineligible to sit in parliament. He said he was checking if his father's birth in Cyprus makes him British, as well as Australian.
"The great irony here is that my father left Cyprus in 1951 in order to get away from British occupation of Cyprus. The suggestion that I could be a British citizen is something that would absolutely horrify my father...," Xenophon told The Australian newspaper.
Xenophon, whose eponymous party holds two key balance-of-power seats in the upper house, has stymied government media law reform plans and criticized the government's reluctance to order a sweeping inquiry into the nation's banking sector.
The citizenship crisis, based on a 116-year-old law which demands an elected lawmaker only have Australian citizenship, has rocked the Australian parliament, ensnaring three government members, three Green party MPs and Xenophon.
Turnbull's one-seat majority in the lower house is in jeopardy after Deputy Prime Minister Barnaby Joyce on Monday said he may be ineligible for parliament due to New Zealand citizenship by descent. His father was born in New Zealand.
Joyce has since relinquished New Zealand citizenship, but is awaiting a High Court ruling, along with several other politicians who believe they may have dual citizenship.
The High Court will begin hearings into Joyce's eligibility on Aug. 24. Should he be ruled ineligible, Turnbull would be forced to rely on independents to pass legislation.
If the lawmakers under a citizenship cloud are ruled ineligible by the High Court there will need to be by-elections held in their seats. If Joyce failed to win re-election in his rural seat, Turnbull would face a hung parliament.
Turnbull's government is struggling in opinion polls and would not wish to be tested at the ballot box at this time.
Australia's foreign minister has accused New Zealand's Labour Party of conspiring to bring down the Australian government by revealing Joyce's New Zealand ancestry.
Under Section 44 of Australia's constitution "a subject or a citizen of a foreign power" is barred from office.
One government senator, Matt Canavan, discovered his mother obtained Italian citizenship on his behalf, while another, Fiona Nash, said she is a Briton because her estranged and long-dead father was born in Scotland.
Turnbull told reporters in Canberra on Friday that he expects the High Court to keep the dual-national MPs in office.
"Section 44 was designed, as the court has said, to prevent politicians having conflicts of loyalties or split allegiances, that's a quote from one of the judgments," Turnbull said.
"I'm very confident that the court will find that Fiona Nash, Barnaby Joyce and Matt Canavan are not disqualified from sitting in the parliament."
(Reporting by Colin Packham in CANBERRA and Tom Westbrook in SYDNEY; Editing by Michael Perry)