The Conservative MP and former jungle celebrity Nadine Dorries is to be investigated by the Parliamentary Commissioner for Standards.
The probe follows a complaint relating to payment for her appearance on ITV's I'm a Celebrity Get Me Out of Here in November last year.
The MP for Mid Bedfordshire, 56, has yet to declare any fee from the reality TV show in the Commons register of members' interests.
She was expected to earn around £40,000 for her 12-day appearance in the Australian jungle.
Last night, Ms Dorries defended herself by posting a series of messages on Twitter.
She said: “If I haven't declared anything, it's because I haven't earnt anything. I must be the most hounded MP in parliament.
“Labour MP John Mann reported me to Standards Commissioner for undeclared earnings. Anything I earn, I declare immediately."
She added “no wonder my bloody hair is falling out!” and predicted the inquiry would find “no case to answer”.
She has earlier told the BBC: "I'm a backbencher not a minister. My personal finances are my business and nobody else's.”
Ms Dorries was suspended by the Conservative Party in November after flying to Australia while Parliament was sitting. She was later reinstated after her stint on the reality TV show came to a premature end as the first contestant to be voted out by the public.
The Parliamentary Commissioner for Standards Kathryn Hudson will now gather evidence on the claims.