Labor believes legal advice shows another federal MP is ineligible to sit in parliament because of their financial interests in a company.
Liberal MP Chris Crewther owns shares in Gretals Australia, which has a research grant from the federally funded Australian Research Council.
Under section 44 of the constitution, MPs are banned if they receive profit from federal funding sources.
"It follows, in our view, that if Mr Crewther holds an indirect pecuniary interest in the ARC funding agreement, then he is incapable of sitting as a member in the House of Representatives, and his seat is thereby vacated," lawyers Matthew Collins and Siohban Kelly told Labor.
The grant was administered through the University of Melbourne.
Concerns have also been raised about Home Affairs Minister Peter Dutton, who has a financial interest in childcare centres, which pass federal funding on to parents.
And new Wentworth independent Kerryn Phelps could also be under a cloud as a GP who passed on Medicare payments to patients.
Both deny any problems.
The latest section 44 doubts come after a dozen MPs were forced to resign when it was revealed they had dual citizenship.