LONDON, DEC. 31 -- A heated controversy erupted today over the release of secret documents that showed U.S. officials were informed of a major British nuclear accident 30 years ago while the public was deliberately kept in the dark.

The documents revealed the full findings of inquiries into the 1957 disaster at the Windscale plant in northwest England, which manufactured plutonium for military purposes.

No one was killed in the accident, which occurred when a fire broke out in the uranium core of the reactor and raged for 16 hours, releasing contaminated particles into the atmosphere. But the true extent of the accident -- the world's worst before the 1986 Chernobyl disaster and believed responsible for dozens of cases of terminal cancer -- was covered up until now.

A spokesman for the Atomic Energy Authority (AEA) said the cover-up was motivated less by fears of public opinion than by fears that the full story could jeopardize Britain's nuclear links with the United States. "The guidance came from a high political level and was primarily concerned with the impact that full disclosure would have had on the Americans' perception," Mark Baker, AEA secretary, said.

At the time, the U.S. Congress was preparing to repeal laws banning nuclear cooperation with America's western allies.

According to AEA officials, then-prime minister Harold Macmillan, of the Conservative Party, feared that American public opinion could lose confidence in British security, jeopardizing more transatlantic exchange of nuclear information. They also said that Britain was involved in a race to establish itself as a nuclear power.

The new disclosures brought strong criticism from the opposition Labor Party. "It is only too typical of Whitehall that they trusted the Americans with information they were not willing to share with the people they are supposed to serve," said Robin Cook, shadow secretary of health and social security.

Left-wing Labor deputy Tony Benn, who served as energy secretary in the late 1970s, said, "The main lesson in all this is that in the field of atomic power and atomic weapons the British people have never been told the truth -- and neither have British ministers."

According to the official documents -- published under regulations that release classified papers after 30 years -- Macmillan ordered a report on Windscale to be rewritten after AEA members said they were concerned about its findings.

The released documents show that not only was the original report suppressed but major efforts were made to ensure that all circulating copies were returned. However, information in the original inquiry was given to members of a U.S. atomic energy team.

Since the Windscale disaster, dozens of people in the area have died and are still dying from leukemia and other cancers. A study about to be published suggests the incidence of leukemia in the region is three times the national average.

The reactors, contaminated with radioactive debris and tons of melted and partly burned fuel, remain highly dangerous but are sealed to contain the radiation. The center, renamed Sellafield, now reprocesses nuclear fuel.