SYDNEY (Reuters) - Australia faces a series of by-elections after the High Court ruled on Wednesday an 11th lawmaker must leave parliament in a test-case decision that prompted four more lawmakers to quit amid a widening citizenship crisis.

The court found unanimously that opposition Labor Party Senator Katy Gallagher was a British citizen at the time of her election, despite her attempts to renounce it, and so ineligible to be elected to the federal parliament.

Dual citizens are blocked from national elected office under Australia’s 117-year-old constitution. The decision prompted four other lawmakers in similar predicaments to Gallagher to resign, triggering by-elections for their lower-house seats.

Gallagher and the other four are not members of Australia’s ruling center-right coalition, keeping its slender majority safe, but the resulting polls will be a litmus test of the public mood and Prime Minister Malcolm Turnbull’s popularity ahead of a general election due next year.

“By-elections are notoriously difficult things,” Nick Economou, a political scientist at Monash University in Melbourne, told Reuters.

He said the Queensland state seat of Longman, held by Labor’s Susan Lamb on a knife-edge margin until her resignation on Wednesday, was a bellwether contest.

“If Labor was to lose that, it’d be a huge fillip for Turnbull,” Economou said.

“On the other hand, if Labor was to win with an increased majority, that would be a sign that there’s a problem for the government,” he said.

Lamb said she intended to recontest her seat and Labor MPs Josh Wilson and Justine Keay, who also resigned on Wednesday, and independent member Rebekha Sharkie, said they would do likewise in their separate electorates.

Dates for the by-elections have not been set officially but could be as early as mid-June.

Gallagher, who said she was disappointed but accepted the court ruling, will be replaced by another Labor politician in the Senate.

“The High Court has adopted the strictest possible reading,” George Williams, professor and Dean of Law at the University of New South Wales in Sydney, told Reuters.

“I’m surprised by the result. I think it delivers a proposition that is unworkable ... because of how hard it will be to comply with these requirements now,” he said.

In a nation in which half the population were either born overseas or have parents who were, the dual citizenship rule has cut a swath through the national parliament.

It has prompted the departure of 15 lawmakers and last year briefly left Turnbull’s Liberal-National coalition clinging to a minority government.

Gallagher, who was born in Ecuador to British parents and was conferred British citizenship, filed paperwork and paid processing fees to renounce it more than two months before being elected in July 2016.

However, she did not get confirmation from the British Home Office that her renunciation had been processed until after she was elected, according to documents she disclosed to parliament last year.