AMs should not compare current domestic political events with Nazi-era Germany, the presiding officer has ruled.
In a Brexit debate, Labour AM Dawn Bowden referred to "Goebbels-like propaganda from the right-wing media".
Presiding Officer Elin Jones warned such comments risked "trivialising the horrors perpetrated by the regime".
It follows a complaint from Welsh Tory leader Andrew RT Davies about "aggressive" language on Brexit. Ms Bowden said she stood by what she said.
Mr Davies had said the comments in a debate in February, which Ms Bowden directed at newspapers backing Brexit in the run-up to the EU referendum, were offensive and in danger of "trivialising one of the most tragic events in modern history".
Replying to him, Ms Jones said: "However passionately members feel about a subject, I do not consider that comparisons with the Nazi regime in 1930s and 1940s Germany are appropriate, and normalising the use of such references risks trivialising the horrors perpetrated by that regime.
"I will continue to monitor this to ensure that such rhetoric does not become a common feature of members' contributions."
Ms Bowden, AM for Merthyr Tydfil and Rhymney, said: "I hear what the presiding officer is saying.
"I don't think my comment was a reference to Nazi Germany in the wider sense.
"It was a reference to propaganda and when you see the way immigration has been reported, that is propaganda.
"I stand absolutely by what I said."
However, the presiding officer rejected a complaint by Mr Davies against another Labour AM, Eluned Morgan, who he claimed "trivialised suicide" when she said in the same debate that Brexit would "slit the wrists of the Welsh economy".
Ms Jones said she wanted to "encourage debate that is vibrant, vigorous and robust and for that reason I do not call members to order for using direct or colourful language to make their point, even when other members do not like the argument being made".
"I do not therefore consider there is anything out of order in the contributions you cite by Eluned Morgan in that debate," she said.