Mammalian cells integrate mitogen and stress signalling before the end of G1 phase to determine whether or not they enter the cell cycle^1-4. Before cells can replicate their DNA in S phase, they have to activate cyclin-dependent kinases (CDKs), induce an E2F transcription program and inactivate the anaphase-promoting complex (APC/C^CDH1, also known as the cyclosome), which is an E3 ubiquitin ligase that contains the co-activator CDH1 (also known as FZR, encoded by FZR1). It was recently shown that stress can return cells to quiescence after CDK2 activation and E2F induction but not after inactivation of APC/C^CDH1, which suggests that APC/C^CDH1 inactivation is the point of no return for cell-cycle entry ³ . Rapid inactivation of APC/C^CDH1 requires early mitotic inhibitor 1 (EMI1)^3,5, but the molecular mechanism that controls this cell-cycle commitment step is unknown. Here we show using human cell models that cell-cycle commitment is mediated by an EMI1-APC/C^CDH1 dual-negative feedback switch, in which EMI1 is both a substrate and an inhibitor of APC/C^CDH1. The inactivation switch triggers a transition between a state with low EMI1 levels and high APC/C^CDH1 activity during G1 and a state with high EMI1 levels and low APC/C^CDH1 activity during S and G2. Cell-based analysis, in vitro reconstitution and modelling data show that the underlying dual-negative feedback is bistable and represents a robust irreversible switch. Our study suggests that mammalian cells commit to the cell cycle by increasing CDK2 activity and EMI1 mRNA expression to trigger a one-way APC/C^CDH1 inactivation switch that is mediated by EMI1 transitioning from acting as a substrate of APC/C^CDH1 to being an inhibitor of APC/C^CDH1.