He said the Polish government had never said he was a dual national. Labor was therefore asking the High Court to rule on a matter the Polish government had not determined.

But Malgorzata Nowotynska from Lexmotion wrote in her advice that “Jason Falinski is [a] Polish citizen from the moment of birth, regardless of the fact that it was not officially confirmed by the Polish government”.

Mr Falinski has previously released Australian legal advice from the firm Arnold Bloch Leibler stating he is not a foreign national but adding the firm “cannot conclusively advise on foreign law” and recommending he get independent foreign advice. Mr Falinski said he had done this, but declined to release that subsequent advice.

Opposition citizenship spokesman Tony Burke said that, on the basis of the advice, Mr Falinski had always been a Polish national.

“This is open and shut … His family came to Australia as Polish citizens on Polish passports,” he said. “Unless he can show he has taken reasonable steps to renounce the Polish citizenship he clearly has, he’s gone.”

A Labor spokesman said Ms Wolski had "confirmed with the Polish government Office of Citizenship that Mr Falinski's father was a Polish citizen who travelled to Australia on a Polish passport".

The latest advice comes amid a tit-for-tat battle over citizenship that carries high stakes because it could rob the Turnbull government of its slender majority, though Mr Falinski's seat is safe.

Labor MP David Feeney resigned last week, triggering a byelection, and the Coalition is calling on Labor's Susan Lamb to step down as well. Mr Falinski said Labor MP Emma Husar would be in the same situation as he is if Labor’s case was sound.

Australian immigration officials recorded Mr Falinski’s grandfather and grandmother as having Polish passports when they arrived in 1958. His father, Stanislaw Falinski, who was 15, had no passport number but was listed as Polish.

Mr Falinski said his father could not be Polish for a number of reasons. He was born in the then Soviet Union after his grandfather, who was Jewish, fled Poland. His grandmother was Russian. Mr Falinski said they were not married when his father was born in late 1943, meaning his father would have been a Russian national.

The Russian citizenship would have been annulled when his father was naturalised in Australia in 1967, according to Mr Falinski’s own legal advice.

He said Polish authorities in 1958 issued temporary travel documents to Jews that might have been incorrectly recorded as a passport by Australian immigration officials.

“In 1958 there was a development in Poland that allowed Jewish people to leave on one-way travel documents as long as they did not return,” he said.

Mr Falinski’s grandparents stated in immigration documents that they were married in the Soviet Union in 1942, but Mr Falinski said they lied about this. They could not have met until at least early 1943, he said.

He said the Polish government does not say he is Polish. He said he inquired about Polish citizenship in 2009 and was told it would take several years to process an application.