Reasonable steps to renounce foreign citizenship may not be enough to escape the constitutional disqualification on a strict reading of the high court’s decision in the citizenship seven case, two constitutional experts have warned.

The interpretation of the court’s unanimous decision leaves MPs who made applications to renounce their citizenship but whose renunciation was not processed or confirmed until after the nomination date vulnerable to challenge.

The Labor MPs Justine Keay and Susan Lamb both applied to renounce their UK citizenship in May, and said they had “taken all reasonable steps” by the 9 June 2016 nomination date, but have refused to say if their renunciation was processed and confirmed before the deadline.

Section 44 of the constitution states that people who are “subjects or citizens of a foreign power” are not capable of being elected. The section applies at the date of a candidate’s nomination for election.

In their unanimous judgment the justices said the section “does not disqualify only those who have not made reasonable efforts to conform to its requirements”.

“Section 44(i) is cast in peremptory terms. Where the personal circumstances of a would-be candidate give rise to disqualification under s44(i) the reasonableness of steps taken by way of inquiry to ascertain whether those circumstances exist is immaterial to the operation of s44(i).”

The judgment states that a person who “retains the status of subject or citizen of a foreign power” at the time of nomination will be disqualified “except where the operation of the foreign law is contrary to the constitutional imperative that an Australian citizen not be irremediably prevented by foreign law” from participation in representative government.

“Where it can be demonstrated that the person has taken all steps are that are reasonably required by the foreign law to renounce his or her citizenship and within his or her power, the constitutional imperative is engaged.”

Asked whether MPs who had applied for renunciation of foreign citizenship but were still dual nationals at the nomination date could be ineligible, University of Sydney constitutional expert Anne Twomey said: “Regarding reasonable steps, I think it is still unclear.

“It is possible to argue that a strict reading of the judgment is that a person is disqualified if he or she is a dual citizen at time of nomination regardless of what steps were taken beforehand.

“The other view is that a person is not disqualified if he or she has taken all the necessary steps to renounce citizenship under the requirements of the foreign country and that delay in processing the renunciation should not prevent a person from being elected.

She said, “It may be necessary for the court to clarify this point”, noting that “the Malcolm Roberts case did not resolve the issue because he had not taken all the relevant steps in advance of nomination”.

The dean of the University of New South Wales law school, Prof George Williams, said he “didn’t disagree” that candidates who had not completely renounced by the nomination date may be vulnerable to challenge.

“We don’t have certainty,” he said. “The court has never had to rule on the situation of what is reasonable with regards to the amount of time before an election for which someone has sought renunciation.”

Williams said what was reasonable may be decided on a “case by case” basis and if candidates renounced foreign citizenship “immediately at the time the election was called” that may constitute reasonable steps to prevent disqualification.

Guardian Australia understands that Labor has received legal advice that the high court’s decision reaffirms Sykes v Cleary that if a person has has taken all steps that are reasonably required by the foreign law they are eligible for election.

University of Queensland professor Graeme Orr backed Labor’s view that it is sufficient to have taken all reasonable steps.

Orr said that it was “by far the better argument”.

He put the chances of a challenge against an MP who had renounced but not received confirmation of renunciation at 75% to 80% probability of failure.

“It would be a very harsh and highly legalistic reading to say they’ve failed to do what was required,” Orr said. “There would be no purpose in saying it isn’t good enough.”

Orr also noted under the strict reading courts would have to determine how long a wait for foreign bureaucracies to process renunciations was reasonable, adding “uncertainty” to the Australian law on eligibility.

In the judgment, the justices said it was not necessary to “multiply examples of requirements of foreign law that will not impede the effective choice by an Australian citizen to seek election to the commonwealth parliament”.

“It is sufficient to say that in none of the references with which the court is concerned were candidates confronted by such obstacles to freeing themselves of their foreign ties.”

The court held Fiona Nash was a British citizen by descent through her father and Malcolm Roberts was also British. They said that Roberts “ceased to be a citizen of the United Kingdom on 5 December 2016, on the registration of his declaration of renunciation of citizenship”.

In a statement Lamb said: “In May 2016, I took all necessary steps to renounce by completing and sending the UK Home Office Form RN ‘Declaration of Renunciation of British Citizenship’ and paying the requisite fee.”

In a statement Keay said she had “renounced British citizenship” by filling the form in on 9 May, 2016 which she sent to the Home Office on 13 May. It was delivered on 23 May and “receipted” on 31 May.

“I had therefore taken all reasonable steps to renounce British citizenship before nominations for the 2016 federal election officially closed at 12 midday on 9 June 2016,” she said.