Cambodia's King Norodom Sihanouk has abdicated, his son has announced.
King Sihanouk is said to be fed up with Cambodia's political infighting
Prince Norodom Ranariddh said Senate President Chea Sim would be interim head of state, but added that he hoped his father would change his mind.
Cambodia's monarchy is not hereditary, and a special throne council is due to meet soon to discuss what happens next.
The king, 81, has abdicated before and threatened to quit again many times. Ill health and political rows may have triggered the latest crisis.
An emotional Prince Norodom Ranariddh announced his father's decision to the National Assembly on Thursday.
"I hope this is not a permanent abdication," he said.
Earlier the king, who has been living abroad and receiving medical treatment, issued a statement saying he was too ill and old to continue in office, and that he wished to abdicate.
"It is up to the Royal Throne Council to decide whether
Prince Sihamoni or who else will be an appropriate successor," the statement said.
It was unclear what triggered the king's announcement, but there has been speculation he is angry about a lack of progress in deciding the process to decide his successor.
According to media reports, his decision may also have been influenced by a letter he received from opposition leader Sam Rainsy.
In it, Mr Rainsy claimed that "violent demonstrations (at least verbally)" against the king were planned
for his return. Mr Rainsy said he would be blamed for them and that his party would be "hastily suppressed".
In a letter read on state television late on Wednesday, the king asked that he be allowed to "retire" because of his fragile health, saying that doctors have detected a "new and serious ailment" in his stomach.
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"I ask all compatriots to please allow me to retire,"
the king's letter said, the Associated Press news agency reported.
King Sihanouk, a mercurial and charismatic figure, has been in poor health for some time.
The move has thrown the country into something of a constitutional crisis, says the BBC's Asia correspondent Andrew Harding.
As Cambodia is not a hereditary monarchy, a nine-member throne council must choose a successor from candidates with sufficiently royal blood.
However, the exact rules have never been worked out and it is still not entirely clear if the king is even allowed to abdicate, since he is meant to rule for life.
Norodom Sihanouk's reign has been as turbulent as his country's history, interrupted by civil war and the Killing Fields of the communist Khmer Rouge when more than 1m of his subjects were killed by starvation or execution.
In recent years he has run his own outspoken website, commenting on Cambodia's endless political power struggles.