Australia has confirmed for the first time that a dual Australian-Israeli citizen who died in a maximum-security Israeli prison had been working for Israel's government.
Officials raised the possibility of a diplomatic spat if Ben Zygier's Australian passport was used for espionage.
Zygier died in an apparent suicide in his prison cell in December 2010, and reports have said he worked for the Mossad spy agency.
Israel's government has refused to release details about the case even after a gag order that barred local media from reporting on it was partially lifted. Australia's government has said only that Zygier, an Australian who emigrated to Israel, had been charged with unspecified security offences.
Australian foreign minister Bob Carr said Zygier was working for Israel's government, but he would not confirm or deny reports that he worked for Mossad.
Mr Carr warned that the case raises serious questions about the use of Australian passports by dual nationals working for foreign governments, saying that if it is confirmed Israel used Zygier's passport for intelligence-gathering purposes, Australia would register "the strongest protest".
"Certainly if Australian passports were misused here, that's something we are forced to take very seriously," Mr Carr told reporters. "No country can allow the integrity of its passport system to be compromised. We can't say whether it happened with Mr Zygier's several passports. I hope the inquiry that takes place ... in Israel can clarify this position."
The mysterious case has strained relations between Australia and its close ally Israel, which suppressed all details of Zygier's imprisonment and death until Australia's national broadcaster broke the story last month.
The Australian Broadcasting Corporation reported that Zygier, who had previously only been known as "Prisoner X", was a Mossad agent who hanged himself in what was supposed to be a suicide-proof solitary confinement cell.
The issue of misused passports is particularly sensitive for Australia. In 2010, an Australian investigation concluded that Israel counterfeited four Australian passports used by a suspected hit squad that murdered a Hamas official in Dubai. Australia retaliated by expelling an Israeli diplomat. Mr Carr said there was no evidence Zygier was involved in the Dubai killing.