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's superannuation fund owns shares in Gretals Australia, which is working on a project with the University of Melbourne.
The university applied for and received a federally funded research grant through the 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.
But Mr Crewther says his own legal advice puts him in the clear.
"I have fully complied with my disclosure obligations and I am confident they do not raise any concerns," he said in a statement to AAP.
"I have also received legal advice from David Bennett QC. On the basis of that advice, I am very confident of my position."
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.