Hundreds of servicemen were poisoned with radiation because the British government did not understand the risks of its nuclear tests in the South Pacific and Australia, a court in London has heard.
The claims were made on the first day of a long-awaited legal battle by more than 800 veterans who are demanding millions of dollars in compensation from Britain's Ministry of Defence.
The veterans from Britain, New Zealand and Fiji claim they were used as guinea pigs by the British government so it could study the effects of radiation fallout from the tests in Australia and the South Pacific between 1952 and 1958.
New Zealanders were among those who watched a series of nuclear explosions at Christmas Island in 1957 and 1958.
Many veterans have suffered a range of illnesses, from cancer to fertility problems and reduced life expectancy, which they blame on the dangerous exposure to high levels of radiation.
They argue that scientific evidence, including a study from Massey University in New Zealand, has linked their health problems with their presence at the nuclear tests and that as a result they deserve compensation.
While the Defence Ministry believes the veterans have left it too long to sue, the veterans want the High Court to decide whether they still have the right to pursue any compensation after 50 years.
A lawyer representing the veterans Ben Browne, QC, told the court on Wednesday the new scientific evidence clearly demonstrated a link between the illnesses suffered by the men and their exposure to the radioactive blasts.
Mr Browne said British government documents showed "things did go seriously wrong" during the tests and the United Kingdom's claims that only a handful of servicemen were exposed to radiation were wrong.
"Firstly it is now apparent that those responsible for the tests did not really understand what risks were entailed," he said.
"One group of men were so badly contaminated by the penetrating radiation that they were found, over days, to produce radioactive urine.
"Another man was found to be 'radioactive above the permissible limit despite four showers and a haircut', indeed his level did not fall to within the permissible level until the following day.
"So the government's own document shows that they did not really know what they were doing and that because of that men were poisoned with radiation," Mr Browne told the court.
Outside court, another lawyer for the veterans, Neil Sampson, said the full liability case would take another two to three years to prepare, if the court orders a trial.
If a compensation hearing can go ahead, the defence ministry could face claims from up to 1,000 individuals, potentially costing millions of pounds.
The initial hearing is expected to last three weeks.