The Telegraph reports:

Foreign spies in Russia have been handed an unexpected gift by officials in the town of Sarov who accidentally posted details of a new top secret submarine on the local administration’s website.

The embarrassing leak followed what was supposed to be a confidential meeting between the commander of the secret submarine and officials in the closed town, which is home to Russia’s main nuclear research facility. Instead, overly assiduous officials wrote a press release that covered the meeting in minute detail, not only naming the prototype vessel’s commander as Capt Sergei Kroshkin but even revealing the project’s code number: 20120. Other technical and tactical specifications were also given, including the submarine’s water displacement of 3,950 tonnes. It was not until the story was dutifully picked up by local newspapers that officials noticed the slip.

The offending press release has now been removed from the website, and Russia’s navy, defence ministry and armament manufacturing industry have all denied the existence of project 20120. Military analysts who have studied the data suggest the new craft, also named the Sarov, is similar in appearance — although much larger — to the fabled Soviet Kilo Class “Turbot” submarine, acknowledged as one of the quietest vessels in the world. Leading Russian newspaper Kommersant said the leaked details suggested that the 20120 contained technology radically different from any other submarine in service. It hypothesised that the Russian navy had revived, perhaps successfully, a Soviet era plan to install a small nuclear reactor on a diesel powered submarine — making it capable of patrolling underwater without surfacing for 20 days.

Current Russian submarines have to surface at least once every three or four days. The revelations are the latest sign of Russia’s rapid rearmament. The country’s defence budget has quadrupled since Vladimir Putin, the Russian president, came to power in 2000. Earlier this year, Russia launched its first new-generation nuclear submarine since the Cold War while yesterday generals said they had successfully tested the world’s largest non-nuclear vacuum bomb — a device they christened “the Father of all Bombs”.