AbS - warning lights come on while driving car?

When ignition is switched on the ABS waring lights come on for 3 seconds and goes off. However, when the car is in motion the lights come on again and this should not happen. This happens at different periods of time when the car is in motion. Can you please advise me how to correct or trace this fault.

Model: Rover 620 SLI - 1997/98 model

email: dnugent14@btinternet.com