Foreign Secretary Boris Johnson has been reprimanded by Commons Speaker John Bercow for using sexist language.
Speaker Bercow intervened - to applause from some Labour MPs - after Mr Johnson referred to shadow foreign secretary Emily Thornberry as Lady Nugee.
Ms Thornberry is married to High Court judge Sir Christopher Nugee - but chooses to go by her maiden name.
Mr Bercow said it was "inappropriate" and sexist to refer to her as Lady Nugee.
He said MPs should be called by their names and not by the titles of their spouses.
Mr Johnson had to be reminded of Ms Thornberry's title by MPs, as he answered a question from a Conservative MP on the Commonwealth, calling her "the Baroness, whatever it is, I cannot remember what it is... Nugee".
Mr Bercow rebuked the foreign secretary, telling him: "We do not address people by the titles of their spouses.
"The shadow Foreign Secretary has a name, and it is not 'Lady something'. We know what her name is. It is inappropriate and frankly sexist to speak in those terms, and I am not having it in this Chamber.
"That is the end of the matter. No matter how senior a Member, that parlance is not legitimate. It will not be allowed, and it will be called out."
Mr Johnson subsequently apologised for his "inadvertent sexism."
Ms Thornberry has occasionally been teased about her formal title by Conservative critics and in February last year complained to Mr Bercow when Theresa May referred to her as Lady Nugee in the Commons.
"Is it in order for the prime minister to refer to a member of this House not by her own name, but by the name of her husband?" the Islington South MP said in a point of order.
"I have never been a Lady and it will be a great deal more than being married to a Knight of the Realm in order to make me one."
Mrs May said: "If the Honourable Lady is concerned about the reference that I made to her, of course I will apologise for that.
"I have to say to her, though, that for the last 36 years I have been referred to by my husband's name."