Former Labor MP Susan Lamb has officially renounced her British citizenship, clearing the way for her to seek a comeback in the by-election for her old seat of Longman, QLD.

"Susan Lamb has informed me that she is not a UK citizen and she is able to be Labor's flag-bearer in the upcoming election in Longman, that's good news," Labor leader Bill Shorten said.  "I am pleased that on this occasion the UK was able to process her application so quickly."

Last week, Ms. Lamb and three other MPs resigned after the High Court ruled Labor Senator Katy Gallagher wasn't eligible to serve because she didn't renounce her British citizenship in time for the 2016 election.  Lamb maintained that she had tried to file the renunciation paperwork with the UK, but her application was rejected because she was unable to obtain a copy of her parent's marriage certificate.

But the UK formally registered her renunciation yesterday after Labor asked officials to reconsider her circumstances without that key document.

"On this occasion, the UK Home Office has been more flexible and we're happy that she has no cloud over her eligibility to serve in the Parliament," said Mr. Shorten.

The other three MPs who stood down - Labor members Justine Keay and Josh Wilson, and crossbench MP Rebekha Sharkie - all promised to contest the by-elections in their former constituencies.