From Longman Dictionary of Contemporary Englishunder no circumstancesunder no circumstances (also in no circumstances British English)NOT DO something used to emphasize that something must definitely not happen Under no circumstances are you to go out. → circumstance
Examples from the Corpus
under no circumstances• Tears must not be allowed to fill their eyes and under no circumstances run down their cheeks.• And under no circumstances are you allowed to vote for me.• Juvenile Court proceedings can be reported but under no circumstances may any child involved in the proceedings be identified.• But under no circumstances comfort the baby, or hold the baby.• That is, under no circumstances must I mention to anyone that I have turned down an invitation to return to Blighty.• Lydon was adamant that under no circumstances would he rejoin the group.• Etiquette demanded that under no circumstances would he change his mind.• The trouble with the proposal was that under no circumstances would the United States give up its ultimate veto on the bombs. 