Labor senator Katy Gallagher has lost her seat after the High Court ruled she did not renounce her British citizenship in time for the 2016 election.

Key points: Gallagher says High Court ruling "deeply disappointing", apologises for "disruption"

Gallagher says High Court ruling "deeply disappointing", apologises for "disruption" Shorten, Wong both say Gallagher is "too good to lose"

Shorten, Wong both say Gallagher is "too good to lose" Decision prompts resignations from Labor's Justine Keay, Josh Wilson and Susan Lamb, and Centre Alliance's Rebekha Sharkie

The court's findings prompted three more Labor MPs and Centre Alliance's Rebekha Sharkie to resign from Parliament.

Ms Gallagher referred herself to the High Court after questions were raised about whether she was eligible to sit in Parliament, but she always maintained she took all reasonable steps to renounce her foreign status before the closure of nominations for the 2016 federal poll, despite the Commonwealth claiming she left her run too late.

The UK Home Office did not register her renunciation until August, more than a month after the election.

Ms Gallagher issued a statement saying she respected the outcome of the High Court case, and apologised to her constituents.

"To the people of the ACT, I'm very sorry that this disruption has occurred to one of your federal representatives," the former ACT chief minister 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.

"I will take the time to talk with Labor Party members on how I can do this over the months ahead."

Her lawyers had argued she could not be held responsible for delays by British authorities and should have been exempt from section 44(i) of the constitution, but the High Court was unanimous in its decision to kick Ms Gallagher out of Federal Parliament.

In its judgment, five of the Justices said the case could be boiled down to two key issues — the finding was that British law did not make it impossible to renounce dual citizenship, and that individuals could not blame delays in the bureaucratic process to renounce their citizenship as a reason to avoid the constitution's dual-citizenship ban.

The court ordered a special recount should be held to fill the vacancy. Union official David Smith is the next candidate on the Labor Senate ticket.

Mr Smith said if and when he took the seat, he had no plans to step aside and create a casual vacancy to allow Ms Gallagher to return to the Senate.

"I think the right thing to do would be to ensure that the decision of the High Court isn't perceived in any sort of way to be undermined," he told ABC Radio Canberra.

"Certainly, I would have no intention of stepping aside."

Four other politicians fall on their swords

The eligibility of Josh Wilson (left) and Centre Aliance member Rebekha Sharkie (right) is under question. ( ABC News: Marco Catalano )

The decision had significant implications for other parliamentarians, with Ms Sharkie, of South Australia's Centre Alliance, and Labor's Justine Keay, Susan Lamb and Josh Wilson falling on their swords over their British ties.

Just after the High Court decision Ms Sharkie insisted her circumstances were "materially different" to Ms Gallagher's, but later resigned from Parliament.

"My paperwork was lodged and received by the UK Home Office before the election was called. My paperwork was returned before the election was held," she wrote in a statement.

Ms Lamb told the House of Representatives chamber that she was "not done yet".

"This is not a valedictory speech," she said.

"I'm putting the Government on notice that while ultimately this decision will lay in the hands of the amazing people of Longman, I intend to be back."

Gallagher 'too good to lose'

Sorry, this video has expired Immediately after the decision, Penny Wong labelled Katy Gallagher "an outstanding senator".

Labor Senate leader Penny Wong sang Ms Gallagher's praises on the Senate floor.

"Katy Gallagher is a woman of great integrity who always acted in accordance with the advice given to her … and always acted in good faith," Senator Wong said.

"Former senator Katy Gallagher is an outstanding senator for the community and people of the ACT; she is an important part of Labor's Senate leadership team and she's too good to lose."

Opposition Leader Bill Shorten said he was "deeply disappointed" for Ms Gallagher and echoed the sentiment of Senator Wong.

Katy Gallagher spent four years as ACT chief minister before stepping into federal politics. ( AAP: Lukas Coch )

"We are a better parliament with Katy in it and a stronger party with Katy in our caucus," he said in a statement.

"Katy is a key part of Labor's Senate leadership team. She is too good to lose from public life — and I know we won't lose her.

"Katy has a lot more to contribute to Labor and to Australia."

Mr Shorten said the party would now consider further implications of the High Court's decision.

ACT Chief Minister Andrew Barr said Ms Gallagher was the most likely Canberra member to sit federally.

"I certainly support her return to Federal Parliament as soon as possible," he said.

Labor's Member for Canberra Gai Brodtmann said Ms Gallagher still had a significant contribution to make to the territory and country, while ACT Liberal senator Zed Seselja said the decision was obviously disappointing for her and wished her well.