Cabinet documents from New Zealand, seen by The Independent on Sunday, show that the US government threatened to pull out of a potential free- trade agreement with New Zealand over its plans for labelling and testing genetically modified food.
The documents also reveal that the US was concerned that curbs on selling genetically modified food in New Zealand could set a precedent for European countries, including Britain.
New Zealand's labelling rules, which were due to become law this week, are set to be reviewed next month.
Opposition MPs have accused the New Zealand government of backing down on GM food under pressure from the US. British MPs say the documents give the first clear evidence of the lengths to which the US will go to defend the American bio-technology industry.
They believe that Tony Blair has privately come under similar pressure from President Bill Clinton and the US government.
They want him to publicly reveal what discussions the British Government has held with the White House, which has close links with Monsanto, about the growth and sale of GM crops in Britain.
MPs are to lodge questions in the House of Commons this week, following the release of the New Zealand documents.
Earlier this month, Mr Blair refused to give details of such talks in a parliamentary answer. "By convention it is not the practice of governments to make information on such meetings or their content publicly available," he said. The New Zealand cabinet documents clearly state that US officials directly intervened in New Zealand's GM testing and labelling plans.
"It now seems clear that the US government is acting on behalf of Monsanto in their attempts to force GM food on unsuspecting consumers worldwide,' said Norman Baker, the Liberal Democrat spokesman on food safety.
"We now need to know how far this pressure has been applied on this country and what the Government's response has been."
The cabinet documents, which are available because New Zealand has a Freedom of Information Act, relate to moves earlier this year to introduce a comprehensive labelling system for food. The proposals include safety testing of GM foods on a case-by-case basis.
The minutes record discussions between the minister of health and the NZ ministries of foreign affairs and trade, the department of the prime minister and the cabinet.
The cabinet minutes, dated 19 February 1998, state: "The United States, and Canada to a lesser extent, are concerned in principle about the kind of approach advocated by Anzfa [part of the Australia New Zealand Food Standards Council], and the demonstration effect this may have on others, including the European Union.
"The United States have told us that such an approach could impact negatively on the bilateral trade relationship and potentially end any chance of a New Zealand-United States Free Trade Agreement."
Phillida Bunkle, health spokesman for New Zealand's Alliance party, said: "Our government has sold the safety of the citizens of this country, ignored what they want and caved in to the might of US corporate power. It is not in the interest of New Zealand to place the further ownership of our food crops in the hands of a few giant US companies."
Since the beginning of this year, British supermarkets have been voluntarily labelling food containing genetically modified elements.
Last week a leaked memorandum from Monsanto revealed that opposition to genetically modified food is growing in the UK and that British MPs and civil servants are highly critical of Monsanto's introduction of such products.
Friends of the Earth, the environmental group, said the New Zealand documents made the US international strategy on GM clear for the first time.
"This is the first clear evidence of the lengths America will go in getting countries to accept unproven genetically engineered foods. This is nothing short of international bullying," said Adrian Bebb, FoE's food and bio-technology campaigner.
"At the same time the UK government continues to give favours to the bio-tech trade despite growing public rejection. Could it be that our own prime minister is listening more to Clinton and Monsanto than the British public? We should now be told."Reuse content