The study was conducted by the National Cancer Institute and the Centres for Disease Control and Prevention, but its publication has been delayed by the US government. However, excerpts of the report were obtained by Tom Harkin, Democratic senator for Iowa, and have been published on a website run by a watchdog group, the Institute for Energy and Environmental Research (www.ieer.org).

The study estimates that an estimated 80,000 people who lived or who were born in the US in the past 50 years have contracted or will contract cancer as a result of American nuclear tests conducted in Nevada and the Pacific ocean, Soviet tests in Kazakhstan and eastern Russia, French tests in the Pacific and British tests on Christmas Island.

Of that number, 15,000 cases are estimated to be fatal. The study reported that everyone living on the US mainland has been exposed to fallout.

"The message is we are all downwinders," said Bob Schaeffer, of the Alliance for Nuclear Accountability, a coalition of pressure groups. He said the report summary obtained by Mr Harkin was dated August 2001, but claimed it had not been made public because of unwillingness by governments to acknowledge the impact of past nuclear testing programmes.

"There is a pattern of denial by both the US and UK governments about the damage done to non-combatants by the nuclear weapons programme," he said. "We want to get this information out so people who live in the areas most affected can get screened and treated."

The IEER's president, Arjun Makhijani, said: "This report and other official data show that hot spots occurred thousands of miles away from the test sites.

"Hot spots due to testing in Nevada occurred as far away as New York and Maine. Hot spots from US Pacific area testing and also Soviet testing were scattered across the United States, from California, Oregon, Washington, and in the west to New Hampshire, Vermont and North Carolina in the east."

The $1.85m (£1.3m) study took two years and measured radioactive isotopes across the US. Lisa Ledwidge, an IEER biologist commended the US government for carrying out an epidemiological study. "It is the only nuclear weapon state to have done so," she said. "But it is not enough to estimate numbers or say you're sorry. The harm is still occurring."

The tests sent plumes of debris into the upper atmosphere where it was swirled around the Earth, depositing highly radioactive isotopes in the form of rain.

"Any person living in the contiguous United States since 1951 has been exposed to radioactive fallout", the study found, "and all organs and tissues of the body have received some radiation exposure."

In the areas worst hit by the fallout, the impact would have been equivalent to receiving one chest X-ray a year, higher than the total recommended for infants or pregnant women. The death toll from the fallout was estimated by comparing the actual incidence of cancer in badly affected areas with national norms.

In the early days of nuclear weapons testing, very little or no notice was given to people living or working nearby. It has long been speculated that the legendary actor, John Wayne, contracted cancer and died as a result of the fallout from a bomb test in Nevada, 100 miles downwind from where he was making a film about Genghis Khan, The Conqueror, in 1954.

By 1980, 91 of the 220-strong cast and crew had contracted or died of cancer. However, the connection between the deaths and the Nevada test was never proven in court.

The latest study was ordered by Congress in 1998 after an earlier study, examining only the dispersal of iodine-131, found that exposure had been considerable across the US. The new study was designed to look into the dispersal of other radioactive elements and to estimate their impact on public health.

"The 1997 report indicates that some farm children - those who drank goat's milk in the 1950s in high fallout areas - were as severely exposed as the worst exposed children after the 1986 Chernobyl nuclear power plant accident. Such exposure creates a high probability of a variety of illnesses," Dr Makhijani said. "Yet the government did nothing to inform the people in these affected areas."