Why dose my handbrake light come on, even tho the handbrake is not pulled?

This has only happend quiet resently, when i drive and i start to brake , say when i come to a set of lights, espesially when i decrease speed rapidly, my handbrake warning light, comes on and when i stop it gose off. It works perfectly fine when i pull the handbrake, but this hapens when the car is in motion.
Ford Ka 1997