The ruling brought on the resignations of three other Labor MPs and a crossbencher facing similar dual citizenship questions, raising the prospect of byelections in marginal seats.

Months of uncertainty ended when the High Court found Ms Gallagher was not eligible to sit in Parliament as she remained a British citizen when nominating for the Senate ahead of the 2016 election.

The court ordered a special count likely to replace her with Professionals Australia's ACT director David Smith, setting up a potential preselection stoush for the territory's Labor branch before the next poll.

Ms Gallagher indicated she could make another run for Federal Parliament, but Mr Smith did not say whether he would stand aside for her at preselection for the Senate.

The former ACT chief minister issued a statement saying she accepted the High Court decision despite her disappointment.

She would not speak to media on Wednesday about the ruling, her efforts to renounce her dual citizenship, or looming preselection questions for ACT Labor.

"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," she said in a statement.

ACT Labor secretary Matt Byrne said it would not be clear for some time whether Ms Gallagher intended to try to return to Federal Parliament.

"It is disappointing but we have to respect the ruling. Katy just put out a statement saying she will take her time [in deciding whether to run for the Senate or the lower house], and talk to Labor party members and we respect that that decision," he said.

Mr Byrne would not be drawn on what Ms Gallagher's decision would mean for Labor's preselection for the new Canberra electorates, nor whether Mr Smith could be bumped down the ticket if she chose to run again for the Senate.

"We're still waiting for the boundaries to be finalised, the national executive sets the timeline for preselections and we will fit our own ones in with that," Mr Byrne said.

"The [third seat] boundaries won't be finalised until the middle of July which means we won't be able to hold preselection until July or August.

"Dave Smith is an experienced union official in Canberra and he will make a good contribution in his time as senator."

Mr Smith told Fairfax Media it was too early to say whether he would step aside to allow her to contest the next election.

ACT Chief Minister Andrew Barr said while the court decision was "disappointing" for Ms Gallagher, he looked forward to seeing her "return to Parliament and continue her work in the future".

"Katy has represented the people of Canberra for 17 years in the ACT Legislative Assembly and more recently in the Senate. I strongly believe that she has more to contribute to public life and am certain that ACT Labor members would support her to make that contribution," Mr Barr said.

Under section 44 of the constitution, members of Parliament can't hold citizenship of another country. In December it emerged that Ms Gallagher was a dual citizen by descent through her English-born father Charles Gallagher when nominating for the Senate.

In light of the High Court ruling, Labor MPs Susan Lamb, Justine Keay and Josh Wilson, as well as South Australian crossbench MP Rebekha Sharkie announced their resignations, but all said they would recontest their seats.

All those MPs were dual British citizens at the cut-off date, but have relied on the same defence as Ms Gallagher, arguing they took reasonable steps to renounce before the deadline.

In Ms Lamb's case, she remains a dual citizen because the British Home Office has never processed her renunciation, saying it needs to see her parents' marriage certificate.

The leader of the opposition in the Senate, Penny Wong, said Labor "will respect the totality" of the High Court decision.

Since the citizenship issue came to light, Ms Gallagher had repeatedly maintained that she believed she was in the clear.

In August 2017, when it was first suggested that her mother might have been an Ecuadorean citizen, having been born in the South American country, she pointed out that the Labor party’s vetting process had found her eligible to sit in Parliament in 2016.

It was her father’s British citizenship that later came under scrutiny. By early November, she was advised that she had taken “all reasonable steps to renounce any entitlement to British citizenship”, and rejected the claims being brought to the fore as a “cheap political stunt” on the part of the Canberra Liberals.

By December, she was adamant that she had done all she could to ensure she had not been a British citizen, but said any referral of her case was in the hands of the Senate.

"Based on all the advice I have available to me, I do not believe that I should refer myself to the Court of Disputed Returns, however, ultimately that will be a matter for the Senate to determine," she said.

In the intervening five months, Ms Gallagher has been silent on the matter, having stepped aside as manager of opposition business in the Senate for the duration of the challenge.