By Bob Howard
BBC Radio 4's Money Box
Mohammed was not aware his car's original owner had had it modified
Drivers who significantly modify their vehicles must alert insurers or risk losing cover, insurers caution.
Changes include anything which affects a car's performance, safety, or the odds of it being stolen or vandalised.
These can include fitting alloy wheels, built in sat-navs or entertainment systems.
The Financial Ombudsman Service says insurers must prove that there has been a significant change to the associated risks for cover to be withdrawn.
Mohammed from Leicester claimed on his insurance after a BMW he bought for £20,000 second hand was badly vandalised, causing £5,000 worth of damage.
He was not aware that some of the car's features were a result of modifications requested by the original owner.
When the insurance assessor discovered modified headlamps, sat-nav and seats it refused to pay out until he agreed to pay an extra £1,000 on his premium and excess.
He agreed, but still thinks it was unfair:
"I shouldn't have been penalised like that as obviously I wasn't aware of it."
Insurers have different policies over what changes they expect drivers to alert them to.
Insurance brokers have seen other examples of controversial decisions.
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:
"We've had silly situations where we've had an insurance company try and throw claim out on the grounds that they had a roof box."
Onus on insurer
Norwich Union says it has several hundred disputes on this issue each year.
It exempts any changes made for an owner before a car leaves the factory, but it requires any alterations after that to be declared.
Nigel Bartram is the company's motor underwriting strategy manager:
"We price the risk on the information we're given and if that proves not to be right we're fully within our rights to void the policy."
Insurers concede that some changes, like some engine modifications, may be impossible for many people to detect.
Unresolved disputes can end up at the Financial Ombudsman Service.
Peter Hinchcliff, its lead ombudsman for insurance, says the onus is on the insurer to explain their actions:
"We'll expect the insurer to explain why the work that was done was relevant for them.
"It's got to be something that makes a difference to the risk they are taking on."
The consumer's group Which? advises anyone buying a second hand car to ask the seller if there have made any modifications and consult a local dealer if they are still not sure.
BBC Radio 4's Money Box was broadcast on Saturday,
31 January 2009 at 1204 GMT.