It happened to an M reg Corsa that I owned a few years ago. As it was only about an inch or so and the spring was still seated securely I carried on using the car for several years. The car was only worth a couple of hundred pounds though. Eventually the car failed the MOT because of corrosion to the bodywork in several places and was sold for spares.
If the car is not too old then I would recommend following Andrew E's advice and get the spring replaced. It is not a massive job.
ChicksFan
February 2015