Retrofit LED lamp for replacing a fluorescent tube lamp in a fixture having a magnetic or an electronic ballast, including electronic ballasts performing a lamp present detection in a detection period at a start-up of the lamp, the retrofit LED lamp comprising: - retrofit lamp connector pin pairs (1; 1', 2; 2') at each end of the retrofit LED lamp, for electrically connecting the retrofit lamp to the ballast of the fixture, - impedances (8,9) connected between connector pin pairs of the retrofit lamp connector pin pairs (1; 1', 2; 2') which correspond with connector pin pairs of filaments of the fluorescent lamp replaced, - rectifying circuitry (4,5) coupled to the retrofit lamp connector pin pairs (1; 1', 2; 2') for rectifying an AC current received from the ballast via the retrofit lamp connector pin pairs (1; 1', 2; 2'), - LED circuitry, coupled to the rectifying circuitry (4, 5), and comprising LEDs for emitting light when energized by the rectified current received from the rectifying circuitry (4, 5), - a detector for discriminating between an electronic ballast and a magnetic ballast on the basis of the voltage and/or current supplied by the ballast to the lamp, - a flicker reduction circuit connectable in parallel over at least a part of the LEDs of the LED circuitry for reducing an AC component in the current through said part of the LEDs so as to reduce the flickering of the emitted light, - a switch circuit (7, 7a; 7b) for establishing the parallel connection of the flicker reduction circuit over the LEDs, and - a switch control circuit (10, 10a) controlling the switch circuit (7, 7a; 7b) and configured to keep the flicker reduction circuit disconnected during a delay period at a begin of a start-up of the LED retrofit lamp and to continuously connect the flicker reduction circuit after the delay period in case the retrofit lamp is energized by a magnetic ballast, whereby the detection period expires before the end of the delay period. - retrofit lamp connector pin pairs (1; 1', 2; 2') at each end of the retrofit LED lamp, for electrically connecting the retrofit lamp to the ballast of the fixture, - impedances (8,9) connected between connector pin pairs of the retrofit lamp connector pin pairs (1; 1', 2; 2') which correspond with connector pin pairs of filaments of the fluorescent lamp replaced, - rectifying circuitry (4,5) coupled to the retrofit lamp connector pin pairs (1; 1', 2; 2') for rectifying an AC current received from the ballast via the retrofit lamp connector pin pairs (1; 1', 2; 2'), - LED circuitry, coupled to the rectifying circuitry (4, 5), and comprising LEDs for emitting light when energized by the rectified current received from the rectifying circuitry (4, 5), - a detector for discriminating between an electronic ballast and a magnetic ballast on the basis of the voltage and/or current supplied by the ballast to the lamp, - a flicker reduction circuit connectable in parallel over at least a part of the LEDs of the LED circuitry for reducing an AC component in the current through said part of the LEDs so as to reduce the flickering of the emitted light, - a switch circuit (7, 7a; 7b) for establishing the parallel connection of the flicker reduction circuit over the LEDs, and - a switch control circuit (10, 10a) controlling the switch circuit (7, 7a; 7b) and configured to keep the flicker reduction circuit disconnected during a delay period at a begin of a start-up of the LED retrofit lamp and to continuously connect the flicker reduction circuit after the delay period in case the retrofit lamp is energized by a magnetic ballast, whereby the detection period expires before the end of the delay period.