Federal Labor wants a Liberal MP to resign, arguing archive material shows his father declared his Polish nationality when he emigrated to Australia.
But Malcolm Turnbull insists backbench MP Jason Falinski "is a citizen only of Australia".
Mr Falinski reiterated he is not entitled to Polish citizenship after three Labor MPs and the Centre Alliance's Rebekha Sharkie were all forced to quit parliament on Wednesday.
Their resignations follow a High Court ruling that ACT senator Katy Gallagher was ineligible to sit in parliament over dual-citizenship.
Mr Falinski was born in Australia, but his father Stanley was born in 1943 in Tokmok, then part of Russia, to Polish-born father Leon and Russian-born mother Maria.
Having previously lodged legal advice with the MPs citizenship register, Mr Falinski on Wednesday uploaded a new letter from the Polish Embassy.
"I can confirm that according to our records you have never had a Polish passport, or a Polish identity card, and you have never applied through our Embassy to relevant Polish authorities to confirm the possession of Polish citizenship," the ambassador wrote on April 27.
"Pleased to have received a letter from the Polish Government confirming what I have always said - I am not entitled to the rights or privileges of Polish citizenship," the MP said.
But Labor is struggling to reconcile the letter with documents held by the National Archives, showing Mr Falinski's father and paternal grandparents all declared their nationality as Polish in 1958.
"If what is in The Australian today is accurate then he should just resign and we have all the by-elections on the one day," Labor frontbencher Tony Burke told Sky News.
Mr Burke said he did not believe the ambassador's letter to Mr Falinski was explicit.
Mr Falinski's lawyers have used his father Stanley's status as an illegitimate child to argue the MP could never have inherited citizenship.
"It continues to be our view that Stanley was born out of wedlock (and therefore would have been considered illegitimate under Polish law). Stanley did not inherit Leon's citizenship on that basis and it is therefore unlikely that you have acquired Polish citizenship by virtue of your paternal grandfather's nationality," they say.
