Lord Sainsbury of Turville held a confidential discussion with three Monsanto executives in his private office at the Department of Trade and Industry on 14 December, three weeks after he attended the first meeting of the Cabinet's Ministerial Group on Biotechnology and Genetic Modification - known as Misc 6.
His meeting with Monsanto, attended by civil servants, raises fresh concerns about the extent of his role in dealing with GM issues within government and the potential conflict with his private business interests.
The day after the Monsanto meeting, Lord Sainsbury chaired a government- sponsored biotechnology seminar with consumer associations, environmentalists such as Friends of the Earth, and one of the Monsanto officials he had met the day before.
John Redwood, the opposition spokesman on trade and industry, last night accused Lord Sainsbury of being misleading over his role in government discussions on GM issues and has called on him to resign.
"Lord Sainsbury has promised us that he has had nothing to do with GM food in government, so I don't see why he is having a meeting with Monsanto on this particular date - the day before the 15 December meeting which he chaired," Mr Redwood said.
"Lord Sainsbury, who is a shareholder and investor in GM companies, had made it clear in some of his statements that because of that he has nothing to do with GM food issues in government," he said.
"We now learn he has had a meeting with Monsanto. So what I want to know from Lord Sainsbury is which story is he going to stick to?"
A statement from the DTI said: "Lord Sainsbury meets numerous companies and other non-governmental organisations in his capacity as Science minister. Last year he agreed to meet Monsanto, at their request, to discuss issues relating to research and development in the biosciences."
At the 14 December meeting, Lord Sainsbury met Ann Foster, Monsanto's director of public and government affairs in the UK, Hugh Grant, president of the company's agricultural division in St Louis, Missouri, and Robert Horsch, general manager of Agracetus, a GM research company owned by Monsanto.
Dr Horsch is one of Monsanto's leading scientists in genetically modified plants and is named on the company's patents controlling the use of herbicide- resistant crops.
Ms Foster said the meeting with Lord Sainsbury included a discussion on GM crops and food. "It's perfectly normal for companies, it's perfectly normal for interested parties to meet ministers," Ms Foster said.