The rules say you should have a cat on your car so if you don't you won't pass your MOT and therefore your car in illegal!

If your car is illegal does it invalidate the insurance even if you declare the fact it is decatted?

I asked the same question b4 but I did not get a definitive answer. I think it is a serious grey area.

Stuart

If the car is not road worthy/illegal then your insurers have the right to decline cover, in the event of an incident you may be uninsured.