Britain is out of step with governments around the world who have compensated nuclear test veterans who fell ill, the High Court has heard.
Benjamin Browne QC, representing 1,000 ex-servicemen, said science has made a link between health and their role in the 1950s tests in the South Pacific.
He said veterans were told to wait for compensation until a link was found and were now being told they were too late.
The Ministry of Defence says it compensates when liability is proven.
Ex-servicemen want compensation for illnesses, including cancer, skin defects and fertility problems, they claim are the result of exposure to radiation during nuclear bomb testing.
But MoD lawyers are trying to derail their claims before they reach a full hearing, by arguing the tests happened too long ago for compensation to be considered.
Mr Browne told the judge: "Time and again, representatives of the government have said that the veterans must wait for compensation since science does not establish a link.
"Yet, when that science does finally become available, the MoD now says that all these claims are far too late.
"This is to be contrasted not only with the UK government's previous attitude where lateness has never been raised, but also with the attitude of many governments around the world who have set up schemes to compensate and are still compensating their veterans as the veterans fall ill."
Mr Browne pointed to the Rowland study of a small group of New Zealand test veterans which "proved that most, if not all, of them suffered genetic effects due to radiation exposure".
He went on to highlight examples he said showed those responsible for the tests did not fully understand the risks.
The UK government recognises the vital contribution service personnel played in the UK's nuclear tests during the 1950s
Ministry of Defence
One explosion resulted in the yield of the bomb being 70 times higher than the minimum yield anticipated, he said.
And on one occasion, a group of men were so badly contaminated by the penetrating radiation that they produced radioactive urine, he added.
He said it was only now, with this new scientific knowledge, that the veterans could proceed with their claims.
It was for the court to decide whether the government should be "entitled to hide behind the time bar raised for the first time in this case so as to snuff out these claims at a preliminary stage", he added.
The US has awarded compensation under the US Radiation Exposure Compensation Act to veterans, including at least one Briton, involved in nuclear testing in the 1960s.
The three-week hearing is expected to hear evidence from 10 veterans who say they were not adequately protected from the blasts and the MoD - at the time the Atomic Energy Authority - should be held responsible.
At the height of the Cold War in the 1950s, Britain carried out a series of nuclear weapons tests in mainland Australia, the Montebello islands off the west Australian coast and on Christmas Island, in the South Pacific.
Veterans who served in the Army, Royal Navy and Air Force, as well as personnel from New Zealand and Fiji, were involved in the tests.
An MoD spokesman said: "The UK government recognises the vital contribution service personnel played in the UK's nuclear tests during the 1950s and understands its obligation to veterans.
"When compensation claims are received they are considered on the basis of whether or not the Ministry of Defence has a legal liability to pay compensation.
"Where there is a proven legal liability, compensation is paid. There is a case ongoing and therefore it would be inappropriate to comment further."
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