This is becoming a fully-fledged transatlantic feud! Miley Cyrus has issued a witty retort to Sinead O’Connor's second open letter of caution.

In her latest tweets, the “Wrecking Ball” singer snarks, "Sinead. I don't have time to write you an open letter cause I’m hosting & performing on SNL this week. So if you'd like to meet up and talk lemme know in your next letter. :)"

After O’Connor posed her first open letter, warning Cyrus against being ‘prostituted’ by her record industry advisers, Miley responded with a nasty message- “Before Amanda Bynes… There was…” along with a reference to O’Connor’s tweets seeking mental health care.

Sinead shot back, "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 mock myself and Amanda Bynes for having suffered with mental health issues and for having sought help."