“One of the reasons the nomenclature is not used is that ‘war’ carries with it a relationship to nation states in conflict with each other and of course terrorism is not necessarily derived from the nation state relationship,” she told the Financial Times. “In some respects ‘war’ is too limiting.”

Ms Napolitano’s comments were the clearest acknowledgement by an Obama official of a widely observed change in language. In March, the White House denied reports that an internal memo had been issued banning the term. But Mr Obama has studiously avoided the phrase, which officials see as legally troublesome and politically counterproductive.