Federal Labor backbencher David Feeney has resigned from the Parliament over dual citizenship, prompting a by-election in his marginal Melbourne seat of Batman.

Key points: Feeney obtained British citizenship through father, but says he revoked it before he was elected

Feeney obtained British citizenship through father, but says he revoked it before he was elected Told High Court last month he still could not find paperwork to confirm renunciation

Told High Court last month he still could not find paperwork to confirm renunciation Resignation prompts Batman by-election, where ABC understands Ged Kearney likely to run for Labor

It is understood union leader Ged Kearney will be announced as the candidate for Labor in the upcoming by-election.

There was a 9.5 per cent swing to the Greens at the last federal election in Batman and Mr Feeney defeated his Greens opponent Alex Bhathal by fewer than 2,000 votes.

Mr Feeney was referred to the High Court in December after he told the Parliament he could not find confirmation he renounced British citizenship inherited through his Northern Irish father.

Section 44 of the constitution prohibits dual citizens from serving in federal parliament, and has already cost seven other politicians their seats in parliament and prompted two by-elections.

A preliminary hearing last month heard Mr Feeney did not yet have the paperwork to prove he revoked his citizenship before he was elected.

Mr Feeney confirmed this afternoon he cannot find the documentation to prove he renounced the British citizenship, despite extensive searches.

"The fact is that after 10 years most records have not been retained," he said.

Mr Feeney said his legal advice was that he could not disprove that he is a dual citizen.

But he said there was evidence he renounced his right to Irish citizenship.

"It of course makes no logical sense that I would do one and not the other," he said.

Two other Lower House MPs, John Alexander and Barnaby Joyce, ran at the by-elections caused by their own dual citizenship, but Mr Feeney said he had decided not to seek re-election.

"This has been an incredibly difficult decision, but both my community and the Labor Party I love deserve a candidate that is able to give them the months and the years ahead, 150 per cent of their effort, their commitment and their passion," he said.

Labor leader Bill Shorten said Mr Feeney had made the right decision, because it meant there would not have to be a High Court case to determine his citizenship.

He thanked Mr Feeney for more than a decade of service to his community and the Labor Party in both the House and the Senate.

Demands for more by-elections

Liberal Cabinet Minister Christopher Pyne has demanded Mr Shorten also force Labor backbencher Sussan Lamb to resign so the by-election for her seat can be held on the same day as the Batman by-election.

Mr Pyne insisted Ms Lamb was a dual British citizen and said it would save taxpayer money if she resigned in the same way Mr Feeney had because it would avoid action in the High Court to determine her citizenship.

A spokesman for Ms Lamb said the MP's eligibility has been confirmed by legal experts.

He pointed out that last year Labor and the crossbench voted to refer a group of MPs including Ms Lamb to the High Court, but Mr Turnbull and the Government voted against it.

Mr Feeney started his political career in the Senate in 2008 but resigned in 2013 to replace veteran Labor figure Martin Ferguson in Batman.

But since then, support for the Greens has been building in the seat, making the margin increasingly tight.

House of Representatives Speaker Tony Smith said he has received the resignation letter from Mr Feeney and is now considering the date for a Batman by-election.

It is expected the poll would be held in March.

Greens hope to repeat success seen in Northcote

Mr Shorten has not confirmed that Ged Kearney will be the Labor candidate for the by-election but said the party will put forward a strong candidate.

Ms Kearney was a nurse for 20 years, and describes herself as a proud union "boss".

She was elected as president of the ACTU in 2010.

Greens leader Richard di Natale said his party hoped to repeat the success it had when it won a by-election last year for the state seat of Northcote, which falls within the Batman electorate.

He criticised Mr Feeney for not resigning earlier.

"I think it stinks he has decided to resign right now — what has changed between the end of the parliamentary year last year and his decision to resign at this time?" Senator di Natale said.

He confirmed Ms Bhatal would again be the Greens candidate in the by-election.