Green’s Dictionary of Slang

1. an informer.

1966

B. Reckord Skyvers Act II: jordan: You’ll go and nark, won’t ya! sylvia: Narker!

2. a police officer.

1959

A. Sillitoe ‘Loneliness of the Long-Distance Runner’ in Loneliness of the Long-Distance Runner (1960) 25: ‘Cancer,’ I’d manage to say to Narker.