Stickers displaying allegiance to sports teams or political opinions could void your car insurance, an investigation has revealed.

Even paint protection products, commonly sold to new car buyers, pose the risk of invalidating a motor policy if not reported to the insurer, investigators discovered.

While every motorist knows that accruing penalty points or buying a car with a more powerful engine are likely to increase their premiums, insurers are more prescriptive than many drivers realise.

The issue is what counts as a 'modification' to the car - and investigators from Auto Express magazine found some surprising examples.

While paint protection may not seem to pose any additional risk to the insured vehicle, it may count as a 'modification' by some insurers.. This means not declaring it is a breach of some policies, with some insurers demanding to know absolutely any deviation from a car’s standard specification.

In the case of stickers, advertising allegiance to a sports team or a political viewpoint may raise the threat of vandalism to the vehicle - with the insurer unaware of the increased risk if the stickers are not declared.

Even official factory-fitted options can be classed by cautious insurers as 'non-standard' if they increase a car’s “value, performance or attractiveness to thieves".

The magazine teamed up with the British Insurance Brokers Association (BIBA) and Ageas insurance to shed light on the thorny issue of what constitutes a modification, with BIBA’s research revealing that some firms subscribe to a definition that classes all optional extras as modifications that must be reported to the insurer.

Hugo Griffiths, Auto Express consumer editor, said: "While some additions to your car are obvious candidates for telling your insurer about, many motorists will be surprised by the strict definition of modification, and how even a sticker in the back window could void your policy.

"BIBA’s work and our investigation revealed, that there are two definitions of a modification available to insurance firms. While one considers a car unmodified as long as it is unchanged from the state in which it left the factory, another stipulates that factory-fit options could make a car modified.

"Failing to declare something as humble as paint protection film or different alloy wheels could void a policy entirely, potentially costing thousands in the event of an incident, and even putting motorists at risk of a £300 fine and 6 penalty points.

"And, in the case of potentially controversial messages on stickers, it’s best to keep your opinions to yourself, or be prepared to share them with your insurer."