When there is no sugar in it but instead sweetners!
This is the latest interpretation of
The Cocoa and Chocolate Products Regulations 2003
SI No. 1659
If there is no sugar but sweetners are used instead then in does not meet the requirements of the "reserved descriptions" which include milk chocolate.
To my mind that is wrong as the key factor should surely be whether there is cocoa in it in some shape or form.
Crazy?