how to disable immobiliser on 1996 renault espace rt2.2i (1991-1996 j/s63)?

my key is often malfunctioning. I afraight one day it will not be working any more. (As my car is 15 years old, and new key is almost 200 euro; replacing the key will not be an option, too expensive for an old car.)
How can I disable by immobiliser in case of emergency?.