Green MP Caroline Lucas has been found to have breached parliamentary rules by giving a tour of the Commons in exchange for a £150 contribution to a fundraising campaign.
An investigation by the Parliamentary Commissioner for Standards began after Tory MP Michael Fabricant complained.
The probe found Ms Lucas had breached the Code of Conduct for MPs.
She apologised for the "completely unintentional" breach and said the money had now been returned.
The offer of a 30-minute "personal guided tour" of the Commons was part of a crowdfunding drive by Brighton and Hove Green Party during last year's general election campaign.
The person who made the £150 contribution later visited the Commons with Ms Lucas after she retained the Brighton Pavilion seat.
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.
Ms Lucas was found to have breached paragraph 16 of the code, which states that the use of publicly funded resources "should not confer any undue personal or financial benefit on themselves or anyone else, or confer undue advantage on a political organisation."
In his complaint, Mr Fabricant said the use of the "House of Commons as an inducement" was an "unfair advantage over other election candidates".
In her response to the commissioner, Ms Lucas said at the time she believed the rules only prevented her offering for auction something that would otherwise be free.
She felt that a "reward offering a personal tour was about someone accessing my time and experience, rather than a public tour which would otherwise be available at a nil cost," she said.
Ms Lucas said she had been contacted by people in five constituencies who claimed they had "either been winners of raffles offering bespoke tours of the House of Commons from their MPs or seen such prizes on offer".
"This would indicate that my reading and understanding of the rules is shared by other MPs," she said.
The commissioner said Ms Lucas's apology and pledge not to repeat the breach was an "appropriate outcome," adding: "The matter is now closed."