Unfortunately, to kick this off, I'm going to direct you towards an EU Regulation :p

In short the regulation:

Promote your maintenance and repair services “from day one†for new vehicles – independent repairers have the right to carry out maintenance services and repair jobs during the warranty period of a vehicle.

Promote competitive spare parts to authorised repairers and vehicle dealers – they have the broad right to use matching quality spare parts supplied by independent parts distributors as an alternative supply channel.

In theory, I could say my new Milltek exhaust system in a perfectly fine spare part as it is of superior quality to the previous? Quality can be proven by the following:

a (self-) certificate for the quality of their parts (e.g. in the packaging, as a separate declaration, or a notice on the Internet).

A manufacturer such as Forge or Milltek already have quality statements on their websites or would easily provide one with the packaging.

Therefore, people saying you have an aftermarket part installed, you go to your dealer, they say 'Modified. Not touching the car and warranty voided.' is actually illegal and against the law.

Am I correct in what I'm saying? I'm thinking this regulation could be heavy bargaining material in any legal wrangle.

I'm not sure that applies; "Quality" in this case (the Milltec) could be argued as "maintaining the OEM specifications". I'm sure you won't claim that the Milltec retains the OEM back pressure and drive-by noise.

If you fit a part that is not to the same design and standard to the OEM part then you have fitted a modification. I presume the named exhaust is not to the same design?

However, they could only argue that they would not cover a warranty claim on a part that is affect by the modified part, so it that case an electrical claim, for example, would probably still be allowed.

I should have said the Milltek exhaust was just a theoretical example. I don't have this after-market mod (not yet anyway )

Regarding the spare part:

it must at least correspond to the original part in terms of its construction, production and functional standards (or it can be of superior quality).

A better example would be a damper/spring suspension after-market solution. Same construction, production and function as a standard but just does the job better.

"Quality" in this case (the Milltec) could be argued as "maintaining the OEM specifications"

I took quality to mean the standard of the after-market part in comparison to the original.

Wouldn't like to try it out, could end up costing lots of money in court.

IF the part was to OE specification, so in the case of the springs the same length and poundage then it would be fine, something tells me that the spec will be modified, either stiffer and/or lower thus making it a modified part rendering the warranty on any associated parts void.

The classic example that 99% of people change (eventually) is the tyres, but providing you keep the width, profile, load index and speed rating the same it is not an issue.

The EU regulation is there to prevent the vehicle manufacturers tying their new vehicle purchasers into their dealer network. During servicing and repair it allows for the use of non manufacturer parts without affecting the warranty, however most manufacturers will put into place a provision that if there is an issue with the vehicle that could have been influenced by the use of non manufacturer parts then it will be the owners responsibility to prove that the parts are equivalent to the manufacturers parts.

A modified part is not equivalent to a manufacturers part, it may offer better performance and be of better quality, but the way in which it works may affect other components. For example take the fitting of a coilover spring/damper fitted in place of a damper, it's a modified part, performs better and of good quality. A year after fitting, the bodywork around the upper mount starts to crack, it's attributable to the extra loads imposed on it by the modified part, not a material or manufacturing defect that has come to light within the warranty period, therefore it is not covered by the warranty.

Warranty covers against defects in materials and workmanship, to be able to provide this warranty within the price of a new car the manufacturer would have forecast and budgeted for unit warranty costs, why should they honour this warranty if the vehicle has been modified beyond it's type approval specifications?