If you receive a warning letter that alleges infringement of an EU you should be extremely careful. The reason is that no response or an unappropriate response may lead to immediate prohibition of the attacked conduct. At least in Germany, a court can issue a preliminary injuction at very short notice. This can have devastating effects on your business. This goes even if it turns out at the end of the day that there was no infringement. Consequently, it is strongly advised that you contact an experienced lawyer as quickly as possible. He/she will assess the case and will make recommendations how to react.