Credit card scandal MP Craig Thomson now has to prove he can even be an MP in Australia.

Thomson has been embroiled in controversy this week after allegations he paid for escorts with a work credit card, and also withdrew cash from a union funded card while on an overseas holiday.

Now he's facing another hurdle with lawyers writing a letter asking him to prove his citizenship.

To be a sitting minister in Australia, you have to renouce your foreign citizenship. Craig Thomson was born in New Zealand and now there's word that he has not done this.

The lawyers are threatening to take the matter to the high court if he doesn't address the citizenship issue.