, who regulates the safety of aftermarket parts in the UK?

Question about OEM regulation. I recently purchased a part for my car a suspension arm from an online parts manufacturer based in Estonia the company is advertising the parts to be OEM specification.

When the suspension arm arrived it was clearly not up to OEM standard, the arm was around 5mm shorter, the metal was of a much thinner gauge. There is obviously a safety issue involved with these substandard aftermarket coming into the UK market, who regulates the safety of aftermarket parts in the UK?

If you want genuine parts, go to the franchised dealer for the brand - if a product is cheaper elsewhere, it's not difficult to work out why.