As many as 250,000 Chinese died between 1937 and 1945 as a result of being exposed to such weapons. More than 2,000 people have subsequently been killed and injured by chemical weapons that were hastily abandoned by the Japanese army at the end of the Second World War. Only two weeks ago, three people in the southern province of Guangdong were hospitalised after inhaling gas that had leaked from discarded artillery shells. Japanese authorities estimate there are 700,000 such weapons scattered around China; the Chinese put the number at two million.
The plant, which is about 20 miles south-east of Hulun Buir city in the far north of Inner Mongolia, was found by a team led by Jin Chengmin from Harbin Municipal Academy of Social Sciences. He said: "It covers an area of 40 square miles. It may be the largest and best-preserved gas experiment site in the world. We've found more than a thousand pits that were used for experiments, as well as trenches and shelters for people and vehicles."
News of the discovery emerged earlier this week, just three days before the 68th anniversary of the start of the Sino-Japanese War, and has further fuelled China's lingering anger over what it sees as Japan's refusal to apologise properly for its army's actions in the war. A sign of how important an issue the war still is to the Chinese came this Thursday when a Beijing museum's exhibition of photos of Japanese army atrocities was opened by Li Changchun, a member of the Standing Committee of the Communist Party's Politburo and one of China's most senior politicians.
Mr Jin's team found the site after consulting the memoirs of a Japanese soldier who had served there and by interviewing elderly residents of the area. One local, a man named Abide who worked at the town's railway station in 1940, recalled seeing special trains carrying Japanese soldiers and prisoners of war arriving at the station and hearing that experiments were being conducted on human beings.
In a report from Xinhua News Agency, Abide said: "I was told that the herders had been driven off the grassland, and many pits had been dug there. In the summer of 1941, if there was a north-westerly wind, people could smell an irritating odour, and many people and animals were affected and died as a result."
Mr Jin expressed his concern about the threat posed by the site: "We can't say there are no bombs left there. We found one bomb in a local family's house. They didn't know it was a bomb and were using it as a tool."
The sparsely populated area was chosen as a testing ground because of its proximity to the Russian border. Mr Jin, who has been researching Japan's biological and chemical warfare programme since 1995, said: "At that time, the Japanese intended to attack the Soviet Union and, because the climate and terrain of Inner Mongolia are similar to Russia's, they wanted to study how these weapons would work in cold conditions."
Under a UN convention, Japan is required to have disposed of all its leftover chemical weapons by 2007. As part of that process, the Japanese will start building a £1bn decommissioning facility at Haerbaling in north-east China this summer, as well as smaller facilities elsewhere.
"Japan conducted chemical and germ warfare in two thirds of the country, but especially in the north, north-east and south of China," said Mr Jin.
Harbin, in north-east China, was the headquarters of the programme and the site of the notorious Unit 731 camp, where Japanese army medical units killed 3,000 prisoners of war and civilians by infecting them with anthrax, cholera and bubonic plague, as well as performing human vivisections. Now, the Chinese want the site of Unit 731 to be granted UN World Heritage status, like Auschwitz and the Hiroshima Peace Memorial.
"It should qualify for World Heritage status," said Mr Jin. "The ruins serve as a permanent reminder of the atrocities Japanese troops committed in China."