Theresa May was forced to apologise after she broke House of Commons rules by mocking one of Labour’s top MPs.
The Shadow Foreign Secretary does not use her official title, which comes from her marriage to the High Court Judge Sir Christopher Nugee.
Critics have used it in the past to point out her lofty family link.
But for Theresa May to use it in the Commons chamber broke the rule by which MPs only address each other by their job titles or constituencies.
Ms Thornberry raised the issue with the Commons Speaker, saying: For the record, I have never been a Lady.
And it will take a great deal more than me being married to a Knight of the Realm in order to make me one!
Mrs May responded: If you are concerned about the reference I made then of course I will apologise for that.
But she then added: I have to say though for the past 36 years I have been referred to by my husband’s name!
Ms Thornberry, well known in Westminster for her heckles, had provoked a flash of anger from the Tory leader during a debate on her trip to see EU leaders in Malta on Friday.
Mrs May said: The concern has been expressed, at this council meeting and not only this council meeting, but at others, about the role that Russia is playing in a number of ways, in terms of the interference that they are taking.
The Shadow Foreign Secretary is shouting at me by you’. Yes, Lady Nugee, by me.
Speaker John Bercow confirmed the Prime Minister had broken the rules.
He said: We refer in this chamber to members by their constituencies or if they have a title, for example shadow minister, by their title.
To refer to them by another name is not the right thing to do but the Prime Minister has said what she has said and I thank her for that, so we shall leave the matter there.