The 60-year-old star was appalled at the eccentric singer for not stopping her fans - known as Little Monsters - from sending threatening messages to her daughter Kelly, after the purple-haired TV personality criticised Gaga's bizarre dominatrix outfit she wore at the 2012 Grammy Awards.
Instead of calming her fans down, Gaga criticised Kelly for ''choosing a less compassionate path'' in her job on TV show 'Fashion Police'.
Sharon published the letter she sent to the 'Born This Way' hitmaker, which read: ''You are nothing more than a publicity-seeking hypocrite. I am calling you a bully because you have 32million followers hanging on your every word and you are criticising Kelly in your open letter. Are you so desperate that you needed to make this public? A word from you would stop all the vile threats.''
Gaga was bullied in school and subsequently she co-founded the anti-bullying Born This Way Foundation. Sharon was left confused at to why someone who publicly supports those who have suffered from bullying wouldn't step up to help Kelly.
She explained in her book, 'Unbreakable', which is being serialised by The Sun newspaper: ''As far as I am concerned, everything that Gaga is trying to stop, ie, bullying, she's perpetuating by not telling her fans to do the same. That's the responsibility.''