Federal Labor is likely to face a series of by-elections early next year with four of its MPs caught up in the citizenship saga.

Four Labor MPs and a Nick Xenophon team MP were all apparently dual citizens when nominations closed last year.

It is a problem for Labor leader Bill Shorten, who had previously insisted the ALP had a strict vetting process.

The four Labor MPs facing questions after Tuesday's release of citizenship documents are Josh Wilson, Justine Keay, David Feeney and Susan Lamb.

Mr Wilson's renunciation of his British citizenship came into effect on June 29, 2016 — 20 days after the June 9 deadline.

The Labor member for Braddon, Ms Keay, had her UK citizenship cancelled on July 11, 2016 — nine days after the election.

Labor MPs Justine Keay and Josh Wilson as well as Nick Xenophon Team MP Rebekha Sharkie are all in trouble. ( Supplied/AAP )

The Labor member for Longman, Susan Lamb, also appears likely to be referred to the High Court.

The British Home Office has said it does not have enough information for her to renounce her citizenship, but legal advice she has presented shows she was a British citizen.

David Feeney, the Labor MP for Batman, said his status was unclear and if he could not find the renunciation documents he would ask to be referred to the High Court.

Sorry, this video has expired David Feeney tells House of Reps he's prepared to be referred to the High Court (Photo: AAP) ( ABC News )

Mr Feeney said on his form he signed documents prepared for him in October 2007 to renounce any British or Irish citizenship and said as far as he was aware, those documents were sent.

But he said the British authorities could not find the documents and it was possible the High Court might have to consider his case.

The Nick Xenophon Team MP whose eligibility is in question is Rebekha Sharkie.

Ms Sharkie said she renounced her British citizenship on April 19 last year, but the British Home office did not return her form of renunciation until June 29, which was after nominations closed.

Ms Sharkie said she had taken all reasonable steps and actions within her power to renounce British citizenship in April, well before nominations closed.

She said in November she had been advised by Prime Minister Malcolm Turnbull she may have to refer herself to the High Court.

By-election headaches loom for Labor

If the High Court rules the MPs were ineligible because they failed to have their renunciations confirmed in time, there will need to be a round of by-elections probably held on the same day next year.

Justine Keay's seat of Braddon is marginal, Mr Wilson's seat of Fremantle is considered fairly safe for Labor and Ms Sharkie won the previously safe Liberal seat for the NXT and holds it by about 5 per cent.

Ms Lamb's Sunshine Coast seat of Longman is extremely marginal — she won it from the LNP's Wyatt Roy last year.

Mr Feeney very narrowly won the seat of Batman in inner Melbourne from the Greens at the last election.

The Greens recently defeated Labor in the Victorian state seat of Northcote, which covers about half of the seat of Batman.

Sorry, this video has expired David Lipson discusses the latest in the dual citizenship saga

The number of cases involving Labor MPs has put pressure on the Federal Opposition.

Shadow Attorney-General Mark Dreyfus said he had never suggested Labor's system was, "watertight".

"What I have done is to say that it's rigorous," he said.

But Finance Minister Mathias Cormann said Labor leader Bill Shorten had been exposed as a "dishonest, sanctimonious hypocrite".

Mr Dreyfus has hit back, naming Liberal MPs including Nola Marino, Jason Falinski, Julia Banks, Josh Frydenberg, Alex Hawke, Arthur Sinodinos, Ross Vasta and Michael McCormack.

He said they all still had questions to answer.

The MPs' citizenship details were contained in the House of Representatives citizenship register released on Tuesday afternoon.

It appears likely Labor senator Katy Gallagher will be referred to the High Court after her citizenship problems were revealed on Monday when Senate documents were released.