Objectives: Research suggests fear of losing control may play a role in multiple anxiety disorders. However, the question of disorder specificity versus disorder relevance has not been examined in fear of losing control. Focusing on cognitive factors related to the experience of OCD and panic disorder, this study aimed to develop an extended questionnaire—the Fear of Losing Control Inventory (FOLCI)—that taps into potentially disorder-specific aspects of feared loss of control. Methods: A pool of potential FOLCI items was administered to a non-clinical sample (n = 603), along with the existing Beliefs About Losing Control Inventory-II (BALCI-II) and other psychological measures. FOLCI and BALCI-II items were subjected to exploratory factor analyses. Exploratory regression analyses examined the relationship between FOLCI and BALCI-II subscales, and symptoms of psychopathology. Results: Six factors were derived, accounting for 71.25% of variance: ‘Agent of Harm’, ‘Thoughts and Feelings’, ‘Self-appraisals’, ‘Timeframe’, ‘Bodily Sensations’ and ‘Escape and Avoidance’. In exploratory regression analyses, the FOLCI and BALCI-II were significant predictors of symptoms of OCD, panic, generalized anxiety, depression and functional impairment, as was the FOLCI’s Thoughts and Feelings subscale, and the BALCI-II’s inflated beliefs about Probability/Severity subscale. Conclusions: Fear of losing control may be relevant across anxiety disorders, and possibly in depression too. Exploratory analyses suggest fear of losing control of thoughts and feelings, and inflated beliefs about the probability and severity of loss of control, are potentially transdiagnostic. Possible domains of disorder specificity include fear of losing control of bodily sensations, and the timeframe within which a catastrophe would occur following a feared loss of control.