The couple - who have been dating since 2009 but separated twice in 2010 - have reportedly gone their separate ways after the former 'Hannah Montana' star's partying ways became a ''problem'' for the 'Hunger Games' actor.
One source said: ''Miley and Liam are done; it's over. She likes to party really hard and can be pretty wild. It became a problem for him.''
Another source added to the New York Post newspaper: ''They have broken up before, and are broken up again now. There was drama because she suspected he had a wandering eye. And she recently tweeted a denial that he cheated.
''While Miley has insisted they are still together, right now they are very, very much apart.''
Liam - who proposed to the 20-year-old singer-and-actress last March - recently flew back to his native Australia to escape his problems and spend time with his brother Chris Hemsworth and the actor's family, wife Elsa Pataky and nine-month-old daughter India Rose.
A source said: ''He needed to get away from Los Angeles and spend some time with his brother and friends.''
However, despite the split friends close to Miley - who was recently photographed without her engagement ring - are hopeful they will reconcile their differences and reunite once again.
One pal said: ''Miley is a fiery character, and they do fight, but she is crazy about Liam. She isn't ready to accept it might be over for good.''