Whistleblower and press freedom activist Edward Snowden has condemned a new law signed on Thursday by Vladimir Putin, saying it's a “dark day for Russia”.

The new anti-terror legislation forces telephone carriers and internet providers to store the private communications of their customers – and turn them over to the government on request.

“Putin has signed a repressive new law that violates not only human rights, but common sense. Dark day for Russia,” he wrote on Twitter.

Mr Snowden is a former employee of the US National Security Agency (NSA) who exposed global surveillance programmes in 2013 through a leak of classified NSA documents.

The whistleblower joined Twitter in September and has more than 2.2 million followers. However, he only follows one account on the social network – that of the NSA.

Russian communication providers may have to spend more than $30 billion to implement the new laws, Mr Snowden added in a second tweet.

“Signing the Big Brother law must be condemned. Beyond political and constitution consequences, it is also a $33b+ tax on Russia's internet,” he wrote.

The communication companies will have to keep a record of their users’ calls, text messages, photos, and internet activity for six months, and store ‘metadata’ for three years, according to the International Business Times.

And messaging services that use digital encryption, such as WhatsApp, Viber and Telegram, could face fines of thousands of pounds if they continue to operate in Russia without handing over their encryption key to the government.

“Even the Soviet Union did not have such an overwhelmingly repressive legislation,” Russian politician and businessman Gennady Gudkov told the Los Angeles Times.

“This is 100 percent a step toward an Iron Curtain.”

The new legislation, which makes “failure to report a crime” a criminal offence, comes into force on 20 July.

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Dmitry Peskov, a spokesman for the Kremlin, told reporters: “The law has been passed along with a list of recommendations to the government designed to minimize potential financial risks.”

“If the law produces any undesired outcomes, the government will introduce measures accordingly by presidential decree.”