Deputy Labour leader Harriet Harman has worn a t-shirt carrying the slogan "this is what a feminist looks like" to Prime Minister's Questions.
The Fawcett Society's campaign t-shirt has been worn by both Ed Miliband and Nick Clegg for promotional pictures - but not by David Cameron.
Last year Green MP Caroline Lucas was told to cover up a t-shirt with an anti-Page Three message on it.
It is not known whether the Speaker had noticed Ms Harman's choice of attire.
A House of Commons spokesman said: "There are many demands on the Speaker's attention whilst he is in the Chair, particularly during Questions to the Prime Minister. If a Member's dress is not in line with the conventions of the House and it is brought to the Chair's attention, he acts accordingly."
Limited rules on members' dress are set out in Erskine May, the official parliamentary rule book. It says MPs should not wear military insignia or uniforms in the Commons and the custom is "for gentlemen members to wear jackets and ties".
However, other standards of dress are expected to be adhered to as a matter of convention.
At the time of the incident involving Ms Lucas, a House of Commons spokesman said: "By convention members are expected not to use their clothing to display slogans or make debating points - members are expected to make their arguments through their speeches.
"It is for the chair of a debate to make a judgement on whether those conventions are being met."
Reports this week suggested that David Cameron had rebuffed on several occasions the magazine Elle's attempts to photograph him wearing the t-shirt worn by Ms Harman today. As well as celebrities, Ed Miliband and Nick Clegg agreed to be pictured wearing the shirt.