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Yesterday, Sinéad O'Connor penned an open letter to Miley Cyrus, in which she advised the "Wrecking Ball" singer, 20, to change the way she's been conducting herself. "I am extremely concerned for you that those around you have led you to believe, or encouraged you in your own belief, that it is in any way 'cool' to be naked and licking sledgehammers in your videos," O'Connor wrote on her website. "Whether we like it or not, us females in the industry are role models and as such, we have to be extremely careful what messages we send to other women. The message you keep sending is that it's somehow cool to be prostituted. … It's so not cool Miley … it's dangerous."

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Needless to say, Cyrus did not take kindly to the remarks, and mocked the "Nothing Compares 2 U" singer, 46, on Twitter, comparing her to Amanda Bynes. Then, O'Connor posted another letter, slamming Cyrus. "Who the f*** is advising you? Because taking me on is even more f*****' stupid than behaving like a prostitute and calling it feminism," she wrote. "You have posted today tweets of mine which are two years old, which were posted by me when I was unwell and seeking help so as to make them look like they are recent. In doing so you mock myself and Amanda Bynes for having suffered with mental health issues and for having sought help."

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Again, Cyrus was unmoved. "Sinead. I don't have time to write you an open letter cause Im hosting & performing on SNL this week," she tweeted. "So if youd like to meet up and talk lemme know in your next letter. :)"

Sure enough, a third letter soon appeared, with O'Connor declining her invitation - and threatening legal action. "Remove your tweets immediately or you will hear from my lawyers," she wrote. "I am certain you will be hearing from all manner of mental health advocacy groups also. It is not acceptable to mock any person for having suffered."

Cyrus has yet to respond.

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