A woman and her daughter were hit by mild food poisoning in western Tokyo in April after eating "komatsuna" -- a kind of Chinese cabbage -- delivered to their home directly from the farm at which it was grown, police said Wednesday.

Analysis of the supposedly organic vegetable by the Tokyo Metropolitan Research Laboratory of Public Health showed high levels of nicotine in the vegetable.

Nicotine is sometimes used as an agrochemical to produce "natural" products, including vegetables.

Police, however, have launched an investigation on suspicion that someone intentionally put nicotine in the produce.

According to police, the 52-year-old woman and her 18-year-old daughter from Inagi, western Tokyo, felt irritation in their mouths after drinking miso soup containing the vegetable on the evening of April 12. They then became nauseous, but their symptoms were not serious.

The woman's husband reported the incident to a public health office the next day, and the office relayed the information to police.

According to the laboratory's analysis, there were 190 parts per million of nicotine in the uncooked vegetable and 0.5 ppm in the miso soup. No nicotine was found in other ingredients used in the soup.

The laboratory said it estimates that there could have been 60 mg of nicotine in the whole 300-gram portion of cabbage -- a potentially lethal amount.

It said it is highly unlikely such a high level of nicotine could have come from agrochemicals alone.

Meanwhile, the farmer denied ever using agrochemicals containing nicotine on the vegetable, which was bagged, boxed and shipped to consumers nationwide.

The farmer added that no similar reports of food poisoning have been received from other consumers.