The 'public dialogue' set up by Government quango the Food Standards Agency is already in disarray after the vice chairman resigned.
Professor Brian Wynne said the FSA was promoting GM 'propaganda' and incapable of carrying out an objective public consultation.
Now the Soil Association, Friends of the Earth, GM Watch and two other non-Governmental organisations have written to the FSA in support of Prof Wynne.
In a letter to Lord Jeffrey Rooker, Chairman of the FSA, they complained that the dialogue is a "silly waste of money".
"As public interest groups who oppose the use of expensive, unproven and environmentally and socially damaging GM technology in farming and food, we do not intend to have anything to do with your GM assessment, and this £500,000 waste of public money, and we will encourage our supporters and others to avoid giving any spurious legitimacy to this exercise," it reads.
Lord Peter Melchett, policy director of the Soil Association, said the public consultation will have no credibility without the input of public interests groups.
"It cannot possibly have an credibility for exactly the reason Professor Wynne said, that the FSA is displaying an institutional bias in favour of GM," he said.
The FSA pointed out that the public dialogue is still at the planning stage and will only go-ahead when the Government decides.
The Department for the Environment Food and Rural Affairs, that is in charge of the decision, said it is still being considered.
“The details of the Government’s GM policy are yet to be decided, and ministers will consider whether to continue with the Dialogue," a spokesman said.
