Green MP Caroline Lucas is under investigation for a possible breach of parliamentary rules for offering a tour of the Commons in exchange for £150, as part of a fundraising drive.
The Code of Conduct for MPs states members must not offer tours of the House in raffles or auctions.
A complaint went to the Parliamentary Commissioner for Standards about the offer last year, the BBC understands.
Caroline Lucas said she did not believe she had done anything wrong.
The offer of a 30-minute “personal guided tour” of the Commons in exchange for a donation was part of a crowdfunding drive by Brighton and Hove Green Party during last year’s general election campaign, to re-elect the Brighton Pavilion MP.
It is understood the person who made the £150 donation visited the Commons with Caroline Lucas earlier this year.
Members of the public can pay for a guided tour around the Houses of Parliament or UK residents can arrange one free of charge through their local MP.
It is understood the Standards Commissioner began an investigation after receiving one complaint about the Green Party offer, but a subsequent complaint about the same issue was also received.
In a statement Caroline Lucas said: “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.”
The Office of the Parliamentary Commissioner for Standards said it did not comment on ongoing matters and would neither confirm or deny that Caroline Lucas was being investigated.