Code Zero

The Thirteenth Doctor classed the appearance of a reconnaissance scout Dalek on 1 January 2019 as a Code Zero emergency, which she attempted to report to Kate, only to be directed to Polly at the UK Security Helpline, who informed her that UNIT operations had been suspended. (TV: Resolution)

UNIT themselves used Code D in reference to the Daleks. (AUDIO: The Dalek Transaction)