Veteran Papua New Guinean leader Sir Michael Somare on Thursday hit out at rival Peter O'Neill, saying he had made the impoverished country "the laughing stock of the Pacific and the Commonwealth".
O'Neill was Wednesday appointed as prime minister -- the third time in less than a year that the 47-year-old has been sworn in as leader.
He won office in August in a vote of lawmakers during the prolonged absence of predecessor Somare, who was recovering from major heart surgery at the time.
But Somare, now 76, later recovered and has contested O'Neill's rule ever since, taking his case to the Supreme Court, which has twice ruled that O'Neill's rise to power was illegal and Somare should be reinstated.
"You have shown no respect for tradition or protocol let alone the rule of law," Somare, who maintains he is the rightful leader, said of O'Neill in emailed comments.
"What shame you all bring to Papua New Guinea," he said.
"Mr O'Neill and his fellow rogues have crossed the line of decency. Your actions will determine your fate in your respective electorates."
PNG is due to go to the polls on June 23, seen as an important election with the country sitting on massive reserves of gas and minerals that analysts say have the potential to double GDP.
Earlier this week, the Commonwealth bloc of nations called on Papua New Guinea to respect the rule of law and the judiciary's independence, while Australia warned PNG was risking its international image.