Mr Rainsy was disqualified as an MP in 2011, after he was convicted of a series of offences, which observers claimed were politically-motivated.
He has been granted a royal pardon and is due to return to Phnom Penh from self-exile in France on Friday.
Speaking from Paris, Sam Rainsy told Sen Lam he's hopeful he will be able to challenge Prime Minister Hun Sen at the ballot box.
Presenter: Sen Lam
Speaker: Sam Rainsy, Cambodian opposition leader and leader of the Cambodia National Rescue Party
RAINSY: Yes, negotiations are ongoing, to ensure that I be a candidate, not only for member of parliament, but also for prime minister. Because as the leader of the Opposition, I am the only real challenger to outgoing prime minister Hun Sen. It's only a small step. Many problems remain to be solved, especially a reform of the Electoral Commission and a reform of the voter list, because the voter list is manipulated by the ruling party to secure victory, even before voting day.
LAM: The UN Special Rapporteur on Cambodia just this week, said that it would be good, if you are allowed political participation in Cambodia. Do you think the Hun Sen government might take note of that?
RAINSY: Of course, but this is only a symbol. We need a number of concrete and important measures to address the problems pointed out by the United Nations.
LAM: But what are your legal advisers telling you? Are you in a good position to register as a last-minute candidate?
RAINSY: Yes, I think this is common sense, because the upcoming election would be meaningless, if the outgoing prime minister, Mr Hun Sen is alone to run for premiership, whereas there's no other challenge, or candidate. As the leader of the Opposition, I am the only real and serious challenger to Mr Hun Sen. Can you imagine, a boxing ring where there is only one boxer? And if Mr Hun Sen pretends, after the match that he is the winner, this would be ridiculous. I think the whole audience would laugh.
LAM: And Sam Rainsy, you arrive in Phnom Penh on Friday, scheduled to arrive on Friday - what will you do first?
RAINSY: I hope to go straight to the royal palace to pay my respects to the His Majesty, the King and to express my gratitude to him. And then, immediately after, I will go to meet Cambodians, all over Cambodia. To visit several provinces a day, in order to cover virtually the whole country in one week.
LAM: About a third of voters in the coming polls are aged between 18 and 30 - how is the CNRP engaging the youth vote - what is your party offering them?
RAINSY: The youth are our main supporters now, because they realise that their future is not with the ruling party, because a large number of youths are unemployed. There is not adequate jobs creation and the ruling party has miserably failed in giving youths decent jobs, so the youths realise that this country needs a change. A change in leadership, in policy, in order to provide decent jobs to a growing number of youths who reach the job market every year.
LAM: You say the Hun Sen government has not done very much for the youth, but isn't it a fact that the capital Phnom Penh is booming and indeed, Cambodia's economy has improved over the past ten years under the stewardship of prime minister Hun Sen?
RAINSY: Yes, but if you look a little bit deeper, you will realise that the development that Cambodia has enjoyed so far, is not sustainable and not equitable either. This development has come at the expense of natural resources, of the environment, of the forests. And another engine of growth is human trafficking, prostitution. This has caused a destruction of the social fabric and on top of that, it's not equitable, in the sense that the growth has benefited only a few number of people, whereas the vast majority of the population remain extremely poor. So, we want a more equitable growth and development for the country, while preserving our natural resources, preserving our social fabric, meaning preserving our future.