Embattled Australian Prime Minister Malcolm Turnbull lost his grip on parliament when another of his ruling MPs resigned Saturday, the latest victim of a constitutional crisis over politicians who hold dual citizenship.

Former tennis star John Alexander became the latest of a number of federal politicians to resign because they may hold dual citizenship in violation of Australia's constitution, and his departure leaves Turnbull without a majority in parliament.

Australia's High Court on October 27 ruled that five other serving members of the Senate or lower house were ineligible due to the prohibition in the country's 1901 constitution.

They included Barnaby Joyce, the deputy prime minister and leader of the rural-based National Party, which rules in a coalition with Turnbull's Liberals.

Alexander said the ruling forced him to examine his own status as the son of a British immigrant, although he insisted there confusion remained over whether he automatically held dual citizenship on that basis.

‘I have always believed that I am Australian and solely Australian,’ Alexander said at a press conference in his Sydney district.

‘In view of recent events and the High Court decision, in my position, I have had to thoroughly examine my situation.

‘I can no longer, with sufficient certainty, maintain the belief that I have held through my 66 years,’ he added.

‘It is my obligation that I must resign and that's what I will do.’

After his resignation, expected to take effect on Monday, Alexander will have to run in a by-election to return to parliament. Joyce similarly needs to run for re-election in his Queensland constituency next month.

Turnbull now controls only 74 of the 150 seats in the House of Representatives, though he can continue to govern with the support of independents.

The main opposition labour Party holds 69 seats.

A number of other MPs from both the Liberal and labour parties are investigating whether they may also hold dual citizenship through their spouses, parents or grandparents.

In a bid to end the uncertainty caused by the dual citizenship crisis, Turnbull announced a plan early this week that would make it compulsory for MPs to formally declare they are not dual citizens and provide evidence to support it.

But the labour party has so far snubbed the proposal.

Alexander played tennis professionally in the 1970s and 80s, at one point rising to world number 8. He also represented Australia in the Davis Cup and captained its Fed Cup team.

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