A controversial French biologist, whose 2012 paper on the alleged dangers of pesticides was withdrawn, has published new claims that the chemicals were many times more toxic than advertised.

Gilles-Eric Seralini's earlier work found that rats exposed to genetically modified maize and the pesticide Roundup developed tumours and other health problems, but his findings were questioned and his paper in the journal Food and Chemical Toxicology finally retracted after his study methods were found lacking.

He told AFP on Thursday that follow-up tests, this time using human cells, showed that Roundup and other pesticides that can be bought over the counter were "between two and 1,000 times more toxic than their main, active ingredient".

This was problematic, said Seralini, as the toxicity of the active ingredient is what determines product guidelines for accepted exposure levels to the pesticide being used.

"There has been a miscalculation of the real toxicity of pesticides," the professor said, claiming his research showed "cells begin to commit suicide" in petri dish experiments after exposure to the chemicals.

Seralini's earlier paper was quashed after it prompted probes of pesticide safety by several national health agencies as well as the European Food Safety Authority (EFSA).

The new study was published in the journal Biomed Research International, co-signed by two of Seralini's colleagues at the University of Caen in France.

It tested three herbicides (Roundup, Matin El and Starane 200), three insecticides (Pirimor G, Confidor and Polysect Ultra), and three fungicides (Maronee, Opus and Eyetak).

Of the nine, eight "were several hundred times more toxic than their active principle," the study said -- and pointed to other "confidential" additions to the pesticide recipe.

Neither Roundup manufacturer Monsanto nor the EFSA could be reached for comment on Thursday.

The findings were welcomed by the French NGO Generations Futures (Future Generations), which demanded full disclosure of all the ingredients of pesticides.