Friends of the Earth has said it will start a legal challenge against the Food Standards Agency (FSA) over the sale of GM rice in UK supermarkets.
GM rice has been banned in the European Union
It claims the agency privately told retailers they did not need to test for contamination of rice by GM products.
GM rice is banned in the EU because of fears not enough research on possible health risks has been carried out.
The FSA said: "It is the responsibility of retailers to ensure the food they put on the market is in compliance."
Friends of the Earth said it had sent a number of rice samples for testing after reports in the US that long-grain rice had been contaminated by a type of GM rice - Bayer CropScience's LLRICE 601 - grown in experimental trials in August.
The European Commission then introduced emergency measures to stop it entering Europe. No GM rice has yet been approved for consumption in the EU.
Friends of the Earth said a leaked memo revealed the FSA had told food retailers and manufacturers in private it did not expect them to carry out tests to see if rice was contaminated, or remove contaminated rice from sale.
GM material was found in two types of own-brand rice for sale at Morrisons, Friends of the Earth said.
The supermarket said it was withdrawing from sale 500g packets of American Long Grain Rice with a best before date of May 2008 and its 1kg pack of American Long Grain Brown Rice with a best before date of July 2008.
A Morrisons spokeswoman said: "Based on information received about tests carried out by Friends of the Earth, we have withdrawn the two products implicated as a precautionary measure."
The pressure group said the FSA had said in the memo it would only be testing for contaminated rice at mills, and any which had been sold to stores or was in warehouses would not be withdrawn. The FSA says the rice poses no threat to human health.
'Poses no risk'
Friends of the Earth says it has written a formal legal letter before beginning action it hopes will lead to a judicial review.
Friends of the Earth's head of legal Phil Michaels said: "The Food Standards Agency's response to this GM contamination incident is scandalous and, we believe, unlawful.
"It has failed to act adequately to prevent illegal GM rice reaching our plates and has failed to provide accurate advice and information as it is required to do by law."
The FSA's spokesman said: "We haven't told retailers not to test, but we haven't required them to test."
She said research by the European Food Standards Authority "does not suggest it poses a risk to health".