As credit gets tighter, you may choose to make changes to upgrade your old car instead of buying a new one. However, a report by the BBC's MoneyBox programme reveals if you make significant improvements, insurers will expect to be told so your premium can be adjusted accordingly. If you fail to do so, in extreme cases you may be left without any cover at all.

Changes include anything which affects a car's performance, safety, or the chances of it being stolen or vandalised. If you make improvements such as a DVD system or built-in satellite navigation, tell your insurer otherwise you could be left without cover.

Insurers concede that some changes like engine modifications may be impossible for a second hand buyer to detect. Nigel Bartram from Norwich Union says it's policies don't require customers to give notice of any modifications carried out by the manufacturer before a car is delivered to its owner, but they can be strict on changes that come after.

"The three key things that we would be looking for there would be anything that would increase the performance of the vehicle from standard, anything that might compromise the vehicle's safety and also anything that would make the car more susceptible to theft because we price the risk on the information we are given and if that proves not to be correct then we are fully within our rights to void the policy. We've done that fairly recently on a car where it was very clear that the vehicle was modified and the customer had to pay for his own repairs which cost him £4,000."

It's not just younger drivers adding things like alloy wheels which insurers want to know about, but also parents putting in home entertainment systems to keep the kids quiet. Even people within the industry say it's a grey
area. Peter Staddon from the British Insurance Brokers' Association says whilst drivers have a duty to disclose important changes, some insurers extend the definition too far.

Peter Staddon, British Insurance Brokers' Association: "We get silly situations where we had an insurance company try and throw a claim out on the grounds that somebody had a roof box. Now the Ombudsman very quickly stepped on that."

Peter Hinchcliff, Financial Ombudsman Service: "Let's take the example of a DVD system that's put in the car. It's one thing for the insurer to say you didn't tell me about it and therefore you are not on cover, I'm not paying for the accident; it's another thing for you to say actually I want a couple of thousand pounds more for the car or for the damage or the theft because I've put this really expensive system in there that I didn't tell you about. We might well say it's reasonable for the insurer to stay on cover for the big loss but actually not to pay out for the additional items that you failed to tell them about."