He is Ta Mok, also known as the Butcher or Chhit Chouen - possibly the cruellest and most violent of the Khmer Rouge commanders who turned Cambodia's green countryside into the killing fields.

The Prime Minister of Cambodia, Hun Sen, has hopes to try Ta Mok for his crimes next month. Many in his shattered country are happy at the prospect. Others, including many of the political leadership and bureaucracy, fear his testimony will unveil their own roles during the time of Pol Pot's genocide.

The unease is not restricted to the small, desperately poor, swampy country of 10 million that is modern Cambodia. For when Ta Mok takes the stand, his lawyers promise, no one will be spared - least of all the Western leaders who, they say, supported the Khmer Rouge despite the Maoist extremists' atrocities being widely known.

The most damaging element, for Britain at least, of Ta Mok's court appearance will be new evidence about how British troops and diplomats helped the Khmer Rouge in their fight for power.

Contacted in his prison cell through an intermediary last week, he confirmed to The Observer that the extent to which London and Washington helped the Khmer Rouge in their fight to control Cambodia would be revealed during his trial. The evidence will contradict statements made by Margaret Thatcher's Government - which authorised the operation at the time.

Ta Mok's lawyer, Benson Samay, said the court would hear details of how, between 1985 and 1989, the Special Air Service (SAS) ran a series of training camps for Khmer Rouge allies in Thailand close to the Cambodian border and created a 'sabotage battalion' of 250 experts in explosives and ambushes. Intelligence experts in Singapore also ran training courses, Samay said.

To allow Ministers to deny helping the Khmer Rouge, the SAS was ordered to train only soldiers loyal to the ousted Prince Norodom Sihanouk, and the liberal democrat former Prime Minister, Son Sann, who were fighting alongside Pol Pot's Communists. However, Samay said the court would be told the Khmer Rouge benefited substantially from the British operation.

'All these groups were fighting together - but the Khmer Rouge were in charge. They profited from any help to the others. If they had won the war outright, then Pol Pot would have been back in charge,' Samay said.

The Khmer Rouge and their allies were fighting against the Vietnamese-backed puppet regime Hanoi had installed after ousting Pol Pot's extremist Communists and exposing the horrors of the killing fields.

In a classic piece of Cold War realpolitik, Britain - prompted by the Americans - appears to have given military assistance to the Khmer Rouge-led coalition, despite knowing of Pol Pot's atrocities, in an attempt to limit the power of the Soviet-backed Vietnamese.

'Thatcher, Reagan, Kissinger - they should all be on trial along with Ta Mok,' Samay said last week. He said the court would also hear that humanitarian supplies for Cambodian refugees in Thailand were diverted to the Khmer Rouge with, he claims, the knowledge of the Americans and the British. The court would also hear, he said, how the diplomatic support offered by London and Washington to the coalition led by the Khmer Rouge was 'a great help and morale booster' for Pol Pot's troops. The coalition retained the Cambodian United Nations seat throughout the Eighties.

Ta Mok's journey from jungle hideout to power to hideout and eventually to prison last May is a powerful symbol of the political tides that have washed over Cambodia in the past decades. In April it will be 25 years since Pol Pot's Chinese-backed Maoist revolutionaries defeated a weak pro-US government and entered Phnom Penh. They themselves were ousted by the Soviet-backed Vietnamese four years later and for 15 years a vicious civil war - fuelled by Cold War politics - racked the country.

The trial of Ta Mok and his Khmer Rouge colleague Kaing KhekIev (nicknamed 'Deuch') - who ran the regime's most notorious torture centre - is a litmus test for this deeply scarred nation. Arguments over the format of proceedings have yet to be resolved - the United Nations and human rights groups fear the trial will be used by the government for political ends or be a sham, or both. But it seems likely it will go ahead nevertheless. Few feel, however, that anyone will be pleased by the outcome.

Not far from the prison where its former commander is being held, the Tuol Sleng torture centre still stands. Its iron beds, manacles and electric cables are intact, though tourists and groups of school children now walk wide-eyed through its cells.

Overlooking the rusting barbed wire are the garish villas of the nouveaux riches who have successfully exploited Cambodia's recent shift towards a new, hugely corrupt, free-market economy. Outside its gates loiter half a dozen beggars - dirty children and disabled victims of the mines that still litter Cambodia's countryside - hoping to beg a few riels (Cambodia's virtually worthless currency) from wealthy farang (tourists).

They know what should happen to Pol Pot's henchmen. 'They should all be punished,' said Pheach Yui, 35, who lost his leg to a mine while fighting against the Khmer Rouge 12 years ago. 'They should all be rounded up and judged and punished for their sins. They should be in jail until they die.'

Yui is likely to be disappointed. There are thought to be 50,000 former Khmer Rouge fighters in government positions. At least five are Cabinet Ministers. Others have been effectively pardoned and live well. They include Ieng Sary, the Khmer Rouge number three and Pol Pot's brother-in-law, Nuon Chea, who was known as 'Brother Number Two' and Khieu Samphan, the movement's one-time Prime Minister.

Even Ta Mok says that they should face punishment. 'I know about only a fraction of what happened,' he told The Observer through an intermediary. 'You should ask Ieng Sary and the others too.' Several key Khmer Rouge commanders are gen erals in the Cambodian army and look untouchable. Even the Prime Minister himself was a Khmer Rouge cadre until being recruited by the Vietnamese.

Ta Mok and 'Deuch' may end up being the only senior Khmer Rouge brought to justice for their crimes. Pol Pot, the architect of the the massacres, died in 1998 and no one else has been arrested or is likely to be.

Though some argue that 'national reconciliation' means forgetting the past, to many the failure to bring the Khmer Rouge killers to justice merely emphasises the cheapness of human life in Cambodia today.

The psychological scars of genocide and war are obvious everywhere. The smallest incident can provoke extreme violence. The crime columns in the press are almost grotesque: three men blow themselves, and a café, to bits playing Russian roulette with an anti-tank mine; a man is murdered in a row over whether the millennium bug is a hoax; a syphilitic farmer kills five children and drinks their blood in the hope of being cured; a chess game ends with one dead, two badly injured. Arguments over land regularly lead to murder.

Attacks with acid have become more common. Last month a government official's wife hideously burnt her husband's mistress by pouring five litres of nitric acid over her while bodyguards held the screaming woman down. Such 'crimes of jealousy' are increasing. Last summer Cambodia's most famous actress was shot dead in the street. The press reported that her murderers had been hired by the wife of the Prime Minister - her alleged lover.

'There is an ingrained culture of might is right,' said one Western diplomat. 'It needs very little to spark off appalling violence.' Armed robbery is common and, as the police are corrupt and ineffectual, people take the law into their own hands. Vigilante killings are rou tine, with even novice monks and art students beating suspected robbers to death. The customs and the military, often with the co-operation of senior members of the government, collude in massive smuggling - of beer, drugs, people, tropical hardwood and the country's archaeological heritage.

Cambodia has lost half its forests in the past 30 years, and the trees are still falling fast. Last year soldiers used heavy equipment to break up 30 tonnes of stone carvings from 1,000-year-old archaeological sites before loading them into army trucks and driving them to Thailand to sell to dealers with rich Western clients. The military have even been reported to have been extorting 'protection money' from those trying to conserve Angkor Wat - Cambodia's world-famous jungle temple complex.

The level of development is appallingly low. Average life expectancy is 52, one in five children dies before reaching the age of five, more than a third of the population live below the poverty line and half the children show the effects of malnutrition. Aids killed 6,000 people last year. The elite's exclusive golf course, on the outskirts of Phomh Penh, charges $20,000 (£12,000) for membership, 80 times the average income.

Even the international community's well-meaning interventions often come unstuck. The UN peacekeeping operation hugely boosted Aids in the country and created a parallel dollar economy. A senior French aid worker was reported to be pimping the orphans in his care.

Recently the partly British-funded Cambodian Mine Action Centre was found to have been clearing land for former Khmer Rouge warlords. They included Chhouk Rin - the commander who, in 1994, kidnapped and killed three Western tourists including a Briton.

Khieu Phen is, like Ta Mok, an old man. He was 30 when the Khmer Rouge came to power and lost his brothers, sisters and brother-in-law in the massacres. He survived the killing fields - where he was forced to work 'day and night' and watched 'sons forced to murder their fathers' - by working harder than everyone else. Now he rides a scooter around Phnom Penh hoping to pick up a passenger and earn enough for a bowl of noodles.

'Sometimes I think we are cursed,' he said. 'Everybody takes from this country. So few people give anything. Everybody betrays us in the end.'