Cambodia's opening of parliament has been thrown into doubt after the two main opposition parties said they would not attend.
Hun Sen has vowed to stay in power even without a coalition
The two parties - the Sam Rainsy Party (SRP) and the royalist Funcinpec - said they would boycott the session in protest at July's general election, which was won by Hun Sen's Cambodian People's Party (CPP).
There are also doubts over whether King Norodom Sihanouk, who was set to oversee the ceremony, would attend.
King Sihanouk said that if the three parties did not attend, Senate president Chea Sim would act on his behalf.
After meeting Prince Norodom Ranariddh - the leader of Funcinpec and son of King Sihanouk - Sam Rainsy said on Friday that neither of the two opposition parties were planning to attend the inauguration ceremony.
"Tomorrow is a very important day, the day the Hun Sen-led
government loses its legitimacy," he said.
But the CPP is still planning to attend.
"Nothing has been changed so far," CPP spokesman Khieu Kanharith told the French news agency AFP. "That means we will inaugurate the parliament alone."
The CPP officially won the general election on 27 July, receiving over 40% of the total votes.
But with only 73 seats in the 123-member National Assembly, it did not gain enough seats for the two-thirds majority needed to rule alone.
Both the Sam Rainsy Party and Funcinpec - which was the CPP's junior coalition partner in the last administration - have refused to join a CPP-led parliament if Hun Sen continues as prime minister.
Both opposition parties claim that the elections were biased towards the CPP.
There are fears that the stalemate situation could lead to a repeat of the violent street protests seen after the last general election in 1998.
Diplomats say that if the crisis is not resolved soon, Hun Sen will probably run a caretaker government until a more permanent solution is found.
But with major legislation needed on the budget, Cambodia's entrance to the World Trade Organisation and the long-awaited Khmer Rouge genocide trials, a stable legitimate administration is a matter of priority.