Bangkok: For years, wealthy and well-connected prisoners in Cambodian jails have been able to buy drugs, alcohol and other items requested in their single-person "VIP" cells, according to human rights campaigners.

Now Cambodia's government has comeup with a novel way to ease chronic over-crowding in 18 prisons accommodating more than 18,000 prisoners across the country.

The underworld's upper-crust will be officially able to buy their way out of the jails and into the luxury of a privately run facility.

Interior Minister Sar Kheng describes the facility as a "hotel or detention centre" providing better accommodation and facilities for inmates who could afford to pay. The plan will generate income for the state, he said.

Work began on Tuesday on the $US4 million ($5.4 million) facility to house 1200 prisoners on 1.5 hectares of land behind the government's notorious Pray Sar prison on the outskirts of the capital Phnom Penh. 

The facility is expected to open in 18 months.

"The one who can afford to stay there we will allow to stay there. There will be 400 people for the first stage," General Department of Prisons spokesman Nuth Savna was quoted by the Phnom Penh Post as saying.

"The price is up to the company how much they want to charge."

Mr Nuth Savna said the facility would have bigger rooms than cells in the country's jails as well as space for exercise and worship, adding no activities would be allowed that bent the law.

"Everything is implemented within government policy and I do not think that there will be massage services or other things," he said.

Malaysian property developer Kunn Rekon Holdings has a 45-year agreement to built and operate the facility, sharing the proceeds with the government.

Soaring numbers of Cambodians convicted for drugs crimes and a log-jammed judicial system where suspects can remain incarcerated for years waiting to face trial, force thousands of prisoners to sleep on concrete floors in squalid conditions in the existing prisons.

Only those with money get access to basic needs such as healthy food, water, daylight and fresh air, according to a 2015 report by the Human Rights group LICADHO.

Released prisoners told how with few exceptions an inmate's financial status dictated how they lived in jail. The report said the going rate for lighting and ceilings fans was up to $US200 a month.

The so-called VIP cells were available to be purchased in a number of prisons.

"In some prisons drugs, alcohol and prostitutes are readily available for those who pay and LICADHO has received reports of some inmates paying up to $US2000 to be transferred to a different cell or prison," the report said.

Corrupt guards in some Asian prisons are known to offer privileges for a price, including in Indonesia and the Philippines.

But Cambodia is the first country to contract the building of a private "VIP" prison.