Campaigners against genetically modified crops in Britain last are calling for trials of GM potatoes this spring to be halted after releasing more evidence of links with cancers in laboratory rats.

UK Greenpeace activists said the findings, obtained from Russian trials after an eight-year court battle with the biotech industry, vindicated research by Dr Arpad Pusztai, whose work was criticised by the Royal Society and the Netherlands State Institute for Quality Control.

The disclosure last night of the Russian study on the GM Watch website led to calls for David Miliband, the Secretary of State for Environment, Food and Rural Affairs, to withdraw permission for new trials on GM potatoes to go ahead at secret sites in the UK this spring. Alan Simpson, a Labour MP and green campaigner, said: "These trials should be stopped. The research backs up the work of Arpad Pusztai and it shows that he was the victim of a smear campaign by the biotech industry. There has been a cover-up over these findings and the Government should not be a party to that."

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Mr Simpson said the findings, which showed that lab rats developed tumours, were released by anti-GM campaigners in Wales. Dr Pusztai and a colleague used potatoes that had been genetically modified to produce a protein, lectin. They found cell damage in the rats' stomachs, and in parts of their intestines.

The research is likely to spark a fresh row about GM crops in Britain. Graham Thompson, a Greenpeace campaigner, said: "It is important because it backs up the research by Pusztai, which was smeared at the time by the industry."

Brian John of GM Free Cymru, who released the findings, said the research was conducted in 1998 by the Institute of Nutrition of the Russian Academy of Medical Sciences and has been suppressed for eight years.

It showed that the potatoes did considerable damage to the rats' organs. Those in the "control groups" that were fed non-GM potatoes suffered ill-effects, but those fed GM potatoes suffered more serious organ and tissue damage.

The potatoes contained an antibiotic resistance marker gene. The institute that carried out the studies refused to release all the information. However, Greenpeace and other consumer groups mounted a protracted legal battle campaign to obtain the report. In May 2004 the Nikulinski District Court in Russia ruled that information relating to the safety of GM food should be open to the public.

The institute, however, refused to release the report. Greenpeace and Russian activist groups again took the institute to court, and won a ruling that the report must be released.

Irina Ermakova, a consultant for Greenpeace, said she had conducted her own animal feeding experiments with GM materials. "The GM potatoes were the most dangerous of the feeds used in the trials ... and on the basis of this evidence they cannot be used in the nourishment of people."

Greenpeace said the Russian trials were also badly flawed. Half of the rats in the trial died, and results were taken from those that survived, in breach of normal scientific practice.