(CNN) More than 65 years since the UK began conducting secret nuclear weapons testing in the Australian Outback, scores of files about the program have been withdrawn from the country's National Archives without explanation.

The unannounced move came as a shock to many researchers and historians who rely on the files and have been campaigning to unseal the small number which remain classified.

"Many relevant UK documents have remained secret since the time of the tests, well past the conventional 30 years that government documents are normally withheld," said expert Elizabeth Tynan, author of "Atomic Thunder: The Maralinga Story".

"To now withdraw previously available documents is extremely unfortunate and hints at an attempted cover-up."

Withdrawal of the files was first noted in late December . Access to them has remained closed in the new year.

Dark legacy

The UK conducted 12 nuclear weapons tests in Australia in the 1950s and 1960s, mostly in the sparsely populated Outback of South Australia.

Information about the tests remained a tightly held secret for decades. It wasn't until a Royal Commission was formed in 1984 -- in the wake of several damning press reports -- that the damage done to indigenous people and the Australian servicemen and women who worked on the testing grounds became widely known.

Indigenous people living nearby had long complained of the effects they suffered, including after a "black mist" settled over one camp near Maralinga in the wake of the Totem I test in October 1953. The mist caused stinging eyes and skin rashes. Others vomited and suffered from diarrhea.

These claims were dismissed and ridiculed by officials for decades -- until, in the wake of the Royal Commission report, the UK agreed to pay the Australian government and the traditional owners of the Maralinga lands about AU$46 million ($30 million). The Australian authorities also paid indigenous Maralinga communities a settlement of AU$13.5 million ($9 million).

While the damage done to indigenous communities was acknowledged, much about the Totem I test -- and other tests at Maralinga and later at Emu Field -- remained secret, even before the recent withdrawal of archive documents.

"The British atomic tests in Australia did considerable harm to indigenous populations, to military and other personnel and to large parts of the country's territory. This country has every right to know exactly what the tests entailed," Tynan said. "Mysteries remain about the British nuclear tests in Australia, and these mysteries have become harder to bring to light with the closure of files by the British government."

Alan Owen, chairman of the British Nuclear Test Veterans Association , which campaigns on behalf of former servicemen, said "the removal of these documents affects not only our campaign, but affects the many academic organizations that rely on this material."

"We are very concerned that the documents will not be republished and the (Ministry of Defense) will again deny any responsibility for the effects the tests have had on our membership," Owen told CNN.

A man in protective clothing at Maralinga with a camera also protected by a plastic cover.

Unclear motives

Responding to a request for comment from CNN, a spokeswoman for the National Archives said the withdrawal of the Australian nuclear test files was done at the request of the Nuclear Decommissioning Authority (NDA), which has ultimate responsibility over them.

The NDA said that "a collection of records has been temporarily withdrawn from general access via The National Archive at Kew as part of a review process."

"It is unclear, at this time, how long the review will take, however NDA anticipates that many of the documents will be restored to the public archive in due course," a spokeswoman said.

Jon Agar, a professor of science and technology at University College London, said the withdrawal "is not just several records but two whole classes of files, many of which had previously been open to researchers at the National Archives."

"These files are essential to any historian of the UK nuclear projects -- which of course included tests in Australia. They have been closed without proper communication or consultation," he added.

Agar shared correspondence he had with the NDA in which a spokeswoman said some files would be moved to a new archive -- Nucleus -- in the far north of Scotland. However the Nucleus archives focus on the British civil nuclear industry, and it is unclear why files on military testing would be moved there, or why those files would need to be withdrawn to do so.

Nucleus also does not offer the type of online access to its records as the National Archives does.

"Why not just copy the files if the nuclear industry needs them at Nucleus for administrative reasons? Why take them all out of public view?" Agar wrote on Twitter

Information freedom

In correspondence with both CNN and Agar, the NDA suggested those interested in the files could file freedom of information (FOI) requests for them.

Under the 2000 Freedom of Information Act , British citizens and concerned parties are granted the "right to access recorded information held by public sector organizations."

FOI requests can be turned down if the government deems the information too sensitive or the request too expensive to process. Under a separate rule , the UK government should also declassify documents between 20 and 30 years after they were created.

According to the BBC , multiple UK government departments -- including the Home Office and Cabinet Office -- have been repeatedly condemned by auditors for their "poor," "disappointing" and "unacceptable" treatment of FOI applications.

Commenting on the nuclear documents, Maurice Frankel, director of the Campaign for Freedom of Information, a UK-based NGO, said it was "worrying that properly released records can suddenly be removed from public access without notice or explanation."

"It suggests that the historical record is fragile and transient and liable to be snatched away at any time, with or without good reason," he added.