The Australian High Court decision ruling Labor senator Katy Gallagher ineligible to sit in parliament has triggered four MPs – including three Labor MPs – to resign over dual citizenship issues.

In a litmus test for both Malcolm Turnbull and Bill Shorten the four MPs will now fight to retain their seats in a “super Saturday” string of byelections in states that will be crucial to the next federal election including Queensland and Western Australia.

While the Turnbull government dials up its rhetoric on Shorten’s failure to force his MPs to resign sooner, Shorten has attempted to frame the looming contests – to be held as early as June – as a chance to cast judgment on the Coalition’s big business tax cuts.

On Wednesday the high court unanimously held that Gallagher was ineligible because she failed to renounce her British citizenship by the nomination deadline before the 2016 election.

Three Labor MPs Josh Wilson (Fremantle, Western Australia), Justine Keay (Braddon, Tasmania) and Susan Lamb (Longman, Queensland), as well as the Centre Alliance’s Rebekha Sharkie (Mayo, South Australia) resigned within hours, accepting the Gallagher case as a precedent because they had relied on the same defence.

All except Fremantle are marginal seats, setting Labor a task to hold Longman and Braddon and giving an opportunity to the Liberal Party to retake Sharkie’s seat of Mayo. Shorten must also defend the seat of Perth after frontbench MP Tim Hammond’s resignation, which will come into effect on Friday along with the other MPs.

After the judgment the four MPs all committed to stand at the byelections and delivered statements outlining the steps they had taken to renounced British citizenship before the 2016 election.

In Gallagher’s case the high court had adopted a strict approach to the constitutional bar on foreign citizens sitting in parliament, clarifying that taking “all steps reasonably required” is only a defence where foreign law “irremediably prevents” the Australian citizen renouncing foreign citizenship.

That would require an “insurmountable obstacle” to renouncing foreign citizenship, five justices said in a joint decision. The court held there was no such irremediable impediment for Gallagher and found that the requirements of UK law could not be described as onerous.

Before the resignations the attorney general, Christian Porter, said that all four MPs were dual citizens of Britain at the election and “must resign”. The government’s leader in the house, Christopher Pyne, added the decision was “unambiguous” in its application to them.

The high court decision’s joint judgment had warned that reasonable steps to renounce foreign citizenship are “not sufficient”.

In the immediate aftermath of the decision Bill Shorten said he was “deeply disappointed” by the loss of Gallagher from the Senate.

Re-emerging after the Labor MPs fell on their sword, Shorten refused to apologise for allowing them to sit in parliament while ineligible, citing the fact they had relied in “good faith” on legal advice, which he refused to release.

Shorten repeatedly argued that the Gallagher decision set a “new precedent” and developed a “stricter test” for the defence of parliamentarians who had attempted to ditch foreign citizenship.

Earlier, Porter had said the decision “is not a reinterpretation or a change of the law, it is a crisp and crystal clear clarification of the law as it was stated in the Canavan decision last year”.

Shorten said he was confident the three Labor MPs would be eligible to stand, meaning Lamb will now have to renounce British citizenship which she has previously claimed to be unable to do.

In a statement Gallagher said she was very disappointed but respected the decision.

“I have always acted on the best available legal advice, which at all times, indicated that I satisfied the eligibility requirements under the constitution,” she said.

“To have my place in the Senate end like this today is very deeply disappointing but I believe that I have more to contribute to public life and I will take the time to talk with Labor party members on how I can do this over the months ahead.”

Gallagher, Sharkie and the three Labor MPs all took steps to renounce their dual British citizenship before the nomination date for the 2016 election.

All except Lamb were successful but the renunciations were not processed and did not become effective until after the deadline. Lamb remains a dual citizen because the UK home office would not process the application without a copy of her parents’ birth certificate.

Section 44(i) of the constitution disqualifies people who are subjects or citizens of foreign powers from sitting in parliament.

But in the citizenship seven Re Canavan decision, the high court held that an Australian citizen should not be “irremediably prevented by foreign law from participation in representative government” where they have “taken all steps that are reasonably required by the foreign law to renounce his or her citizenship”.

Gallagher’s Senate seat will now be filled in a recount election by the second Labor candidate for the Australian Capital Territory, David Smith.

Smith told Guardian Australia he would keep the seat because “we can’t afford to have any interruptions in the representation of the ACT in the Senate” and it was “not a great look” to resign for Gallagher to resume her seat because it would be “seen to be trying to get around the high court decision”.

The ACT is set to be given a third lower-house seat in a redistribution, which would give Gallagher the option of contesting the preselection for the No 1 spot to take back her Senate seat at the next election, or shift to the third lower-house seat.

Sharkie committed to run for Centre Alliance despite acknowledging an approach from the Liberal Party to run for the governing party.

Porter and Pyne ducked questions about possible reform to section 44, despite Liberal senator Linda Reynolds, who is chairing an inquiry into the section, warning a referendum will be necessary to fix it.