The group wants the government and supermarkets to tighten up monitoring. "Intake of excessive amounts of pesticides can lead to acute intoxication while lengthy exposure can cause chronic poisoning that affects the reproductive and nervous system," Greenpeace food safety assistant campaigner Chow Yuen-ping said.

"Many pesticides have been identified as potential carcinogens. Some can be passed from mother to the fetus during pregnancy, and to infants through breast feeding."

Different pesticides consumed together can create a "cocktail effect" that can multiply their adverse effects up to a hundred times, Chow said.

Greenpeace collected 55 vegetable samples from the two supermarket chains between November 2005 and March.

Chow said the findings were "alarming." More than 70 percent of samples collected contained pesticide residues, and 30 percent exceeded international standards.

Banned pesticides were also found in some samples. In one tomato sample from ParknShop, five pesticides were found, including Lindane, Chow said. Lindane is a chemical used to kill lice and was banned in Hong Kong in 1991.

One choi sum sample from a Wellcome store contained 240 times more than the acceptable European Union standard for levels of pesticide residue. "As Hong Kong's biggest fresh vegetable retailers, both supermarket chains should make sure their produce complies with Hong Kong standards and is 100 percent safe for consumers," Chow said. "Neither banned pesticides nor excessive pesticide residues are acceptable in the vegetables we eat."

Peter Johnston, ParknShop general manager of quality, food safety and regulatory affairs, admitted six of the 28 vegetable samples collected by Greenpeace in his stores did contain "very low levels of pesticides."

This was due to "environmental contamination primarily from river water used for irrigation downstream from farms using such pesticides or from adjacent farms" Johnston wrote in a response to Greenpeace.

One pak choi sample contained a legal pesticide that was "marginally over" the standard and this was due to "a minor procedural error" in which workers mistakenly harvested the vegetable two days earlier than allowed after the pesticide had been applied. "Corrective action has already been taken," he said.

In a separate statement ParknShop criticized Greenpeace Monday for not releasing results of tests on vegetables sampled at wet markets.

"According to ACNeilsen, over 75 percent of consumers buy vegetables from wet markets, we feel that Greenpeace is irresponsible to publicly announce only part of their test results," ParknShop said.

The chain said its tests followed World Health Organization guidelines while Greenpeace chose to use EU standards. There were discrepancies between the two, "so it is really about what standards Hong Kong should follow," the chain said.

A Wellcome spokesman said the company had terminated contracts with a Guangdong supplier of contaminated produce.