Sinead O'Connor has turned on Miley Cyrus in a second open letter after the US star replied to her original message.

As previously reported, the iconic singer wrote a letter to Cyrus recently in which she questioned the motives of her team and the direction her career is heading, adding that she is a talented artist.

The 'Wrecking Ball' star replied by posting an image of old tweets from O'Connor asking to see a psychiatrist, with Cyrus comparing her to troubled celebrity Amanda Bynes. She added:

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

This has prompted O'Connor to reply in disgust, saying: "Miley…Really? Who the f*ck is advising you? Because taking me on is even more f*ckin’ stupid than behaving like a prostitute and calling it feminism. 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."

O'Connor then threatened legal action if Cyrus's tweets about her aren't removed, before adding: "Furthermore you posted a photo of me tearing the pope’s photo...as if to imply insanity...by doing so all you have achieved is to expose your staggering ignorance. I suggest you read The Philadelphia Report, The Boston Report, all the reports which will illuminate for you why that action of mine remains sane and valid. By mocking it you mock every child who suffered sexual abuse at the hands of priests and had it covered by the Vatican. You could really do with educating yourself, that is if you’re not too busy getting your tits out to read."

O'Connor has since written a third letter to Cyrus, further criticising the singer's tweets and asking for a public apology.