The threat is being described by WHO insiders as tantamount to blackmail and worse than any pressure exerted by the tobacco lobby.

In a letter to Gro Harlem Brundtland, the WHO's director general, the Sugar Association says it will "exercise every avenue available to expose the dubious nature" of the WHO's report on diet and nutrition, including challenging its $406m (£260m) funding from the US.

The industry is furious at the guidelines, which say that sugar should account for no more than 10% of a healthy diet. It claims that the review by international experts which decided on the 10% limit is scientifically flawed, insisting that other evidence indicates that a quarter of our food and drink intake can safely consist of sugar.

"Taxpayers' dollars should not be used to support misguided, non-science-based reports which do not add to the health and well-being of Americans, much less the rest of the world," says the letter. "If necessary we will promote and encourage new laws which require future WHO funding to be provided only if the organisation accepts that all reports must be supported by the preponderance of science."

The association, together with six other big food industry groups, has also written to the US health secretary, Tommy Thompson, asking him to use his influence to get the WHO report withdrawn. The coalition includes the US Council for International Business, comprising more than 300 companies, including Coca-Cola and Pepsico.

The sugar lobby's strong-arm tactics are nothing new, according to Professor Phillip James, the British chairman of the International Obesity Taskforce who wrote the WHO's previous report on diet and nutrition in 1990. The day after his expert committee had decided on a 10% limit, the World Sugar Organisation "went into overdrive", he said. "Forty ambassadors wrote to the WHO insisting our report should be removed, on the grounds that it would do irreparable damage to countries in the developing world."

Prof James was called in by the American embassy in Geneva "to explain to them why they were suddenly getting an enormous amount of pressure from the state department to have our report retracted". The sugar industry, he discovered, had hired one of Washington's top lobbying companies.

The sugar lobby was unsuccessful that time, but now, he says, "we are getting a replay, but much more powerfully based, because the food industry seems to have a much greater influence on the Bush government".

Since his 1990 report, the International Life Sciences Institute, founded by Coca-Cola, Pepsi-Cola, General Foods, Kraft and Procter and Gamble, has also gained accreditation to the WHO and the UN's Food and Agriculture Organisation.

At one point, says Prof James, "I was asked not to send any more emails about any of the dietary aspects of health that related to sugar. I was told that within 24 hours of my sending a note, the food industry would be telephoning and arranging dinners."

Aubrey Sheiham, professor of dental public health at University College, London, Medical School, said he also encountered the strength of the sugar lobby when he was one of the experts involved in putting together an EC guideline called Eurodiet.

"I wrote the sugar part of that," he said. "When we met in Crete [in June 2000], the sugar people said if the 10% [limit] was in, the whole report would be blocked. I remember we went into a huddle with various people and some of the diplomats, and we were meeting in people's bedrooms and saying, how can we work around this?"

In the end, he said, they worked out that a recommendation that nobody should eat sugar more than four times a day was equivalent to a 10% limit. But he considered the committee had been bullied.

The Sugar Association objects to the new report having been published in draft on the WHO's website for consultation purposes, without what it considers "a broad external peer-review process". It wants a full economic analysis of the impact of the recommendations on all 192 member countries. In the letter to Dr Brundtland, it demands that Wednesday's joint launch with the Food and Agriculture Organisation be cancelled.

The report, Diet, Nutrition and the Prevention of Chronic Diseases, has already been heavily criticised by the soft drinks industry, whose members sell virtually everywhere in the world, including developing countries where malnutrition is beginning to coexist with the obesity common in affluent countries.

The industry does not accept the WHO report's conclusion that sweetened soft drinks contribute to the obesity pandemic. The Washington-based National Soft Drink Association said the report's "recommendation on added sugars is too restrictive". The association backs a 25% limit.

The WHO strongly rejects the sugar lobby's criticisms. An official said a team of 30 independent experts had considered the scientific evidence and its conclusions were in line with the findings of 23 national reports which have, on average, set targets of 10% for added sugars.