Cambodia's newly returned opposition leader cannot be a candidate in upcoming polls, election authorities ruled Monday, rejecting his application to stand against strongman Premier Hun Sen.

Sam Rainsy, who was greeted by huge crowds on Friday after his return from self-imposed exile in France, "did not fulfil the conditions in the process of registering candidates", National Election Committee (NEC) president Im Sousdey said in a letter.

"Regarding the request to register your excellency's name, the NEC cannot include it based on the... electoral law," said the letter, in response to Rainsy's application on Sunday to be added to the list.

NEC officials have previously suggested that it was unlikely the opposition leader, who was removed from the electoral register late last year, would be able to stand in the July 28 polls under the current law.

Rainsy has reportedly warned of a backlash if he is barred from standing.

He is seen as the main challenger to Prime Minister Hun Sen, who has ruled Cambodia for nearly three decades.

Until he received a surprise royal pardon this month, Rainsy had faced 11 years in jail if he returned to Cambodia after he was convicted in absentia on charges that he contends were politically motivated.

Following his return home he has already hit the campaign trail to spearhead his party's efforts to loosen Hun Sen's grip on power.

In an interview with Radio Free Asia Friday the French-educated former banker warned that demonstrations might break out if he is not allowed to stand.

"If I can't participate, after the elections all the Cambodian people will protest and the whole international community will condemn the result and regard this as a sham election," Rainsy was quoted as saying.

No one from his opposition Cambodia National Rescue Party (CNRP) was immediately available for comment on the NEC decision.

The United Nations' special rapporteur on human rights in Cambodia, Surya Subedi, last week urged Cambodia to let Rainsy play a "full part" in politics.

The United States welcomed Cambodia's decision to pardon Rainsy earlier this month, with the State Department urging Phnom Penh "to allow for his meaningful and unfettered participation in the elections".

In a letter to the president of the NEC on Sunday, Rainsy asked whether it was possible to find a way "to reinstate my name in the voter list of NEC and the official list of MP candidates".

He requested to contest in southern Kandal province where Hun Sen is also standing as a candidate.

Rainsy, who fled Cambodia in 2009, was found guilty in his absence of charges including inciting racial discrimination and spreading disinformation.

Despite exile, he has remained active in Cambodian politics and recently joined with former rivals to create the CNRP in an effort to finally unseat Hun Sen.

Hun Sen is one of Southeast Asia's longest-serving leaders. His Cambodian People's Party (CPP) won the last two polls by a landslide amid allegations of fraud and election irregularities.

His government is regularly accused of suppressing political freedoms and muzzling activists. In May Hun Sen said he would try to stay in power for another decade.