Military veterans from New Zealand, Australia, Fiji and Britain have won clearance to sue the British Ministry of Defence for radiation exposure during nuclear tests in the 1950s.
Justice David Foskett said in the High Court in London the turning point in the case was cutting-edge research by New Zealand scientist Dr Al Rowland.
Th UK government is now exposed to a potential compensation bill running into hundreds of millions of dollars if the 1000 veterans of tests in the South Pacific and Australia between 1952 and 1958 prove radiation exposure has caused illnesses including cancer and chromosome damage, the Guardian newspaper reported.
The MoD argued that the group had waited too long to lodge its claim and was excluded under limitations regulations.
But giving his verdict in the High Court in London today, Justice Foskett said a recent scientific study involving veterans in New Zealand had provided new evidence of the potential health impact of the tests.
This was "crucial and pivotal" for any potential case against the MoD, he said.
Five of the 10 individual test cases he had considered were thus permitted, while the other five should be allowed to proceed as well, on grounds of fairness.
The MoD would be allowed to appeal, the judge said, but he urged ministers to consider a settlement rather than drag out legal proceedings further.
The veterans took part in the programme on the Australian mainland, Monte Bello islands and Christmas Island between 1952 and 1958 and have claimed that is the reason they suffer cancer, skin defects and fertility problems.
Many of them are terminally ill and seven have died since the hearing at London's High Court in January.
New Zealand Nuclear Test Veterans Association chairman Roy Sefton was among more than 500 Kiwi servicemen exposed to a series of nuclear tests in the South Pacific in 1957 and 1958, code-named Operation Grapple.
Although the association was independent of the claimants, who include "a bit under 100" New Zealanders, Mr Sefton said it had played a notable role in getting the case to court.
"About five years ago we were involved in setting up with people in Britain to try to get some lawyers and start the case," he said.
"The details have changed since then, but we have continued to provide help in any way possible."
In 2001, the association commissioned a Massey University study of genetic damage in the veterans who were at the nuclear tests, and the results, released in 2007, have since become a crucial piece of evidence for the claimants.
The study, led by Dr Rowland, a Massey molecular scientist, compared the frequency of chromosome translocations in a group of 50 nuclear-test veterans with a control group of 50 men of similar ages and lifestyles.
Chromosome translocations, which could cause illnesses such as blood cancers and cataracts, occurred when a chromosome 'broke' and the unique genetic material within was mixed with other chromosomes, Dr Rowland said.
While the control group had an expected frequency of 10 translocations per 10,000 cells, the frequency among veterans was nearly three times higher at 29 translocations per 10,000 cells.
Dr Rowland was "absolutely certain" the increased genetic damage and associated illnesses in the veterans was due to their exposure to the nuclear tests as the scientists had accounted for any other potential factors in the control group.
The association's lawyer, Gordon Paine, who has provided a link between the British lawyers and Kiwi veterans, said New Zealand had played a crucial role in the lawsuit getting to court.
"I might be a bit partisan, but I think New Zealanders have played a huge role," he said.
"In fact, if it wasn't for the NZNTVA, the lawsuit might not have taken place in Britain."
The Australian Federal Government has stalled for years on whether to pay compensation, citing it was awaiting for a ruling to be made by the British High Court since the tests were done by the British Government.
"This is a brilliant day," one veteran said outside the London court as he embraced a colleague.
In earlier hearings, the court heard the British Government and military administration actively withheld details of the dangers of atomic testing in the Pacific not only from the 25,000 servicemen who took part but also the Australian Government of the day.
The British atomic testers ordered military aircraft and ships to pass close to the mushroom clouds, while on land, soldiers stood close to the drop zone with little more than their uniforms.
Tests on their bodies later showed they had been exposed to dangerous levels of radiation but the effects were not known or at least understood.