Green MP Caroline Lucas is being investigated for allegedly breaching House of Commons rules by charging money for a tour of Parliament.
The Brighton Pavilion representative said on Wednesday that she does "not believe" she has done anything wrong after the Parliamentary Commissioner for Standards investigation came to light.
An online fundraiser by her local party in the run-up to the December general election offered a 30-minute tour guided by Ms Lucas and put its worth at £150.
Ms Lucas said in a statement: "I'm aware that a case against me has been brought to the commissioner, and an investigation is currently under way.
"I do not believe I have done anything wrong.
"I have been advised that the commissioner's investigations are confidential so it would not be appropriate for me to say anything more at this time."
Ms Lucas's local paper reported that 69-year-old Hove resident Stuart Bower complained to the commissioner regarding the crowdfunding page saying it was "unethical".
"Charging a fee and boosting your income to take people around the House of Commons is bringing the office into disrepute," he added.
Parliamentary Commissioner for Standards Kathryn Stone wrote back saying she was not launching an inquiry into his complaint because "an overlapping matter is already under inquiry", the Argus reported.
Ms Stone's office would not confirm or deny that the investigation had been launched.
Guided tours of Parliament can be booked online at a price of £26.50 for adults, but UK residents can arrange free tours by contacting their constituency MP.
