Cabinet minister Mathias Cormann is adamant Queensland Labor MP Susan Lamb's dual citizenship is a clear cut breach of the constitution.
Ms Lamb tearfully revealed in parliament this week she could not prove her UK citizenship has been renounced because her mother walked out on her when she was six years old.
Senator Cormann said the circumstances were obviously sad, but paring back the emotions, Ms Lamb had confirmed she was still a British citizen.
"That is a clear and objective breach of the constitution," he told Sky News on Friday.
The finance minister said there was a clear process available to renounce British citizenship.
"It is just not believable that there wouldn't have been a way to obtain the relevant documentation in order to complete the renunciation process for the United Kingdom," Senator Cormann said.
"It's sad, but she's in breach, and she is not entitled and not eligible to be a member of the House of Representatives. It is as clear cut as that."
Ms Lamb told parliament she was unable to obtain a crucial copy of her parents' marriage certificate - which the UK Home Office requires to complete its paperwork - because of her estrangement from her mother.
