Jakarta: Cambodia's government has one last chance to reverse its slide from democracy into full-blown dictatorship, according to a United Nations-appointed expert.

In a bluntly worded message, the UN's special rapporteur on human rights in Cambodia, Rhona Smith, a British professor, said "no election can be genuine if the main opposition party is barred from taking part".

"Those who currently rule the country have one final opportunity to reverse the current trajectory, and return instead to the constitutional path of multi-party democracy and genuine elections – ensuring a level playing field for all political parties," she said.

Smith's comments come ahead of the July 29 national election and as Cambodia's National Election Committee began the two-week process of registering political parties for the poll.

But according to the Phnom Penh Post, Cambodia's strongman leader Hun Sen told a gathering of students on Monday there was no way the main opposition representation, the Cambodia National Rescue Party, would be allowed to contest the election.