Russia's Gulag History Museum says a researcher has discovered a secret Moscow directive in 2014 ordering the destruction of some of the last remaining documents on Soviet-era prisoners -- a move it described as "catastrophic" for historians.

The discovery by Russian researcher Sergei Prudovsky, who has posted online the evidence he provided to the Moscow museum, has alarmed historians and prompted a Russian human rights body to intervene, media reported on June 8.

As many as 17 million people were sent to the Gulag, the notorious Soviet prison camp system, in the 1930s and 1940s, and at least 5 million of them were convicted on false testimony. The prison population in the sprawling labor camps peaked at 2 million people.

Case files of the gulag prisoners were often destroyed, but their personal data was kept on registration cards, which are still held by police and intelligence agencies.

The museum said the newly disclosed classified order in 2014 instructed Russian officials at those agencies to destroy the registration cards of prisoners who had reached the age of 80 -- which now would include almost all of them.

Prudovsky told AFP he discovered the secret order when he contacted authorities in far eastern Magadan, where Soviet prisoners once mined gold, and was told a prisoner's record card had been destroyed under an "official order" from 2014.

"I found out absolutely by chance the record cards were destroyed," said Prudovsky, who specializes in researching camps in Russia's far east.

"I submitted a request. I was interested in the fate of one person, whether he had survived in the camps. I found out that there exists an order for internal use."

The reply from the local Interior Ministry branch, which Prudovsky posted on Facebook, says record cards are only stored for a limited time under an order given in 2014 to the ministry, the FSB security service, and other agencies with Soviet-era archives.

The cards contained information such as when prisoners entered camps and moved between them, as well as what happened to them in the end -- whether they died or were released, researchers said.

Under President Vladimir Putin, Russian authorities have moved to downplay the horrors of the Soviet imprisonment of millions, including the many political dissidents sent to the gulag camps.

They in particular have played down Soviet ruler Josef Stalin's terror, during which millions of people were killed or convicted and sent to camps, instead hailing Stalin for building a new economy and helping the Soviet Union win World War II.

Prudovsky said he passed the letter he received from the Interior Ministry to the gulag museum, which then contacted the Presidential Rights Council, an influential advisory body at the Kremlin.

The head of the council, Mikhail Fedotov, told RIA Novosti news agency that the destruction of such cards would be "barbarism," but that he hoped in this case "simply some mistake happened." He promised to raise the question with officials.

"We will always defend preserving archive records; they contain very important historical information," Fedotov told Interfax news agency.

"It is crucial, as it is a means to counter the falsification of history," he said. "When there is a document, it is almost impossible to falsify. And if there is no document, anything can be invented. This is why all documents of that time must be preserved if possible."

Aleksei Makarov, a researcher at Memorial, the country's top rights group that honors the victims of Soviet repression, said the news came as a surprise.

"We heard about this from Sergei Prudovsky when he received the letter. This is an order for internal use, not published, which doesn't get discussed with the public or with Rosarkhiv," he said, referring to the state archive agency.

It is unclear how many cards have been destroyed, Makarov said.

"It's impossible to understand the scale of what happened," he said. "They could have acted to fulfill it in a half-hearted way.... Unfortunately we don't know how bad the picture is."

If the destruction has happened on a mass scale, "this will make our work more difficult," he said.

A researcher at the gulag museum, Aleksandr Makeyev, told Interfax that so far the problem had only been found in Magadan and the museum wanted to find out if it affects other regions.

U.S. historian Steven Barnes, associate professor of Russian history at George Mason University, told AFP that he had not personally encountered the issue in his gulag research, but added that "any destruction of extant materials related to the history of repression is deeply disturbing."

While most of the gulag central-administration archive has been put on microfilm and stored outside Russia, "a large number of documents remain off-limits to researchers and subject to the whims of the Russian political system for their long-term preservation," he said.

The report has prompted outrage in the Russian historical community and beyond.

Vladimir Zhirinovsky, leader of the nationalist Liberal Democrat Party, said on social media on June 8 that historical "archives should be opened to the public, not destroyed," and that Russians should be able to know the truth about their past.

With reporting by AP, RIA Novosti, Interfax, and AFP