Prokop was riled by one of Bynes' bizarre tweets on Saturday (15Jun13), in which she wrote, "If you're not hot I don't care about you."
The 22 year old replied, "'If you're not hot I don't care about you...' look in a mirror... Stop getting work done. Save what's left."
Things turned nasty between the pair when the Hairspray star targeted Prokop's co-star and girlfriend Sarah Hyland, claiming she had an "ugly face".
The disgruntled actor hit back, writing, "My girl is more successful than you ever will be... enjoy the long road downhill disgusting crack head... It's sad Amanda Bynes was a child actress who kids looked up to. Now she's just a lost soul in a plastic face. get help."
Undeterred, Bynes replied, "I like her ugly face! She isn't as pretty as me! You need to put up two comparable professional photos! She's not a model I am... also new pics because surgery helps me! I need more to make my nose smaller! I'll put our pics side by side to prove my point!"
Prokop pulled out of the Twitter war with one last message, which read, "stop getting work. You were beautiful. Me and Sarah looked up to you. Now you are looking more like a pornstar."
The online argument is the latest round for Bynes, who has criticised the looks of Miley Cyrus, Drake, Lance Bass and Rihanna in recent weeks.
The tweets come amid concerns for the actress' health following her recent arrest for drug charges and erratic behaviour.