My car leaks oil whenever i start up my engine?

I have a 1998 ford contour (2 liter engine) with a really bad oil leak. I go through two 946 ml bottles a week.
Odd because it only leaks only when i start up my engine, oil leaks from the back of engine (kind of hard to explain where because i don't have proper jacks to lift up car for me to examine)
I have a timing belt that needs to be replaced, can that be why? If not does anyone have any suggestions of what it may be? Once again oil leaks lots when i start up my engine. Usually a lot when engine is not started for hours. Leak is starting to get worse!

Use a flashlight and inspection mirror and find that leak, it will likely be at the top of the oil slick on the rear of your motor.

That's almost two liters per week. Why don't you find the leak and do something about it?