The federal opposition in Australia has referred allegations that Labor backbencher Craig Thomson used a union credit card to pay for an escort agency to the New South Wales Police.
Police say they are looking into the matter to see whether they need to start a formal investigation.
Fair Work Australia is also carrying out an investigation.
The ABC reports that if Mr Thompson is forced to resign, the move could bring down the federal government.
Mr Thomson has admitted authorising payments on his Health Services Union credit card to pay for prostitutes in 2005, but said another man used the card and forged his signature.
At the time, Mr Thompson was secretary of the HSU, the union that represents health sector employees.
In documents tendered to the New South Wales Supreme Court prior to a defamation action by Mr Thompson against Fairfax newspapers, it was alleged Mr Thomson spent nearly $A30,000 on personal expenses and almost $A40,000 on costs towards his successful bid for the central coast seat of Dobell.
Mr Thomson has denied the accusations, saying another person has repaid $A15,000 to the union.
But Shadow attorney-general, Senator George Brandis, says either way, criminal charges should be laid against someone.
''It is a crime in New South Wales to use somebody else's credit card for your own personal expenses,'' he said.
Asked if it was the job of the union to call in the police, Senator Brandis said:
''I'll leave it for your listeners to form their own conclusions about why the union mightn't be pursuing Mr Thomson.
''He was, remember, the national secretary of that union before his election to parliament.''
Prime Minister Julia Gillard has twice pledged her support for Mr Thomson in Parliament and says he has her confidence.
The ABC reports a conviction of 12 months or more is required to disqualify a person from holding a seat in parliament.
Mr Thomson holds his seat in the Central Coast of New South Wales with a majority of 5%.
The BBC says that given the Gillard government's deep unpopularity, the Liberals would be expected to win a by-election there.
If Labor loses that seat, it also loses its parliamentary majority.