Your vehicle starts to decrease in value from the moment you buy it, so there is often a big difference between the price you paid for it and its current market value.
If your vehicle is written off or stolen, third party and comprehensive insurance policies will only cover the cost of replacing it at its current market value, not its original purchase price.
Guaranteed asset protection (GAP) insurance covers the difference between your vehicle's purchase price and its current market value. GAP can be bought in addition to, but is not a substitute for, your main motor insurance policy.
There are a number of different GAP policies available and coverage varies between insurers. For more information see the ABI's guide to making sense of GAP Insurance (pdf 1.3MB).