Prison Camp 22 Google Earth There are fears that up to 20,000 may have been allowed to die of disease or starvation in the run-up to the closure of the camp at the end of last year.

The suspicion has emerged from a newly-released report by the Committee for Human Rights in North Korea (HRNK) detailing the situation in penal colonies as Kim Jong-un consolidated his power after taking over as leader from his father, Kim Jong-il who died in 2011. Now the group that is demanding an inquiry into their fate.

The Washington-based organisation gleans information from defectors from the North, including former guards and the occasional survivor of a prison camp, as well as examining satellite imagery.

It focused much of its attention on Camp 22, a vast compound that sprawled across more than 770 square miles, making it larger than London.

The report, North Korea's Hidden Gulag: Interpreting Reports of Changes in the Prison Camps, reveals that two camps have been shut down in the last year but that 130,000 individuals are still being held in penal labour colonies across the country.

"Through this vast system of unlawful imprisonment, the North Korean regime isolates, banishes, punishes and executes those suspected of being disloyal to the regime," the report states.

"They are deemed 'wrong-thinkers', 'wrong-doers', or those who have acquired 'wrong-knowledge' or have engaged in 'wrong-associations'."

Detainees are "relentlessly subjected to malnutrition, forced labour, and to other cruel and unusual punishment," the report says, with thousands more forcibly held in other detention facilities.

"North Korea denies access to the camps to outsiders, whether human rights investigators, scholars, or international media and severely restricts the circulation of information across its borders," the study added.

At Camp No. 22, in North Hamyong Province, in the far north-east of the country, the prison population shrank dramatically in the months before its closure, probably in December 2012.

Reports suggest that a severe food shortage meant that little was passed on to inmates and that numbers dwindled rapidly from 30,000 to 3,000.

Defectors told investigators that as many as 8,000 prisoners may have been transferred to other camps in North Korea's network of gulags, but there are no suggestions that any inmates were released - implying that they may have succumbed to a harsher than usual prison regime.

"North Korea's 2009 currency devaluation (whereby camp authorities were reportedly unable to purchase food in markets to supplement the crops grown in the camps), combined with bad harvests, resulted in the death of large numbers of prisoners after 2010," the report states.

"If even remotely accurate, this is an atrocity requiring much closer investigation."

A United Nations commission of inquiry held hearings in Seoul and Tokyo late last month to examine reports of human rights abuses in the North, including the abduction of foreign nationals, although Pyongyang insists that it respects the human rights of its citizens and refused to allow members of the commission to visit specified sites.

Roberta Cohen, co-chairman of HRNK, called for the International Red Cross to be granted access to the camps as soon as possible.

"An accounting of the fate and whereabouts of all of North Korea's political prisoners, including those missing and those who have died in detention, should be of highest priority to the UN commission of inquiry and the entire international community," she said.

The organisation has stated that it fears the North Korean regime will attempt to erase evidence of atrocities or eliminate surviving prisoners.

Very few North Koreans who have managed to escape from prison camps and to freedom outside the country's borders, but those who have tell of terrible suffering.

Inmates - who can be imprisoned for life, along with three generations of their families, for anything deemed to be critical of the regime - are forced to survive by eating frogs, rats and picking corn kernels our of animal waste.

Activists say that as many as 40 percent of inmates die of malnutrition, while others succumb to disease, sexual violence, torture, abuse by the guards or are worked to death. Men, women and children are required to work for up to 16 hours a day in dangerous conditions, often in mines or logging camps.

Anyone sent to a North Korean labour camp is unlikely to ever leave again, analysts say, while a failed attempt to escape brings execution.

A recent report by South Korea's National Human Rights Commission suggests that the majority of inmates were caught attempting to flee the country in search of food or work, instead of being incarcerated for their political beliefs. Others were detained after being overheard praising South Korea.