MOSCOW - Builders tearing down an enormous Moscow hotel next to the Kremlin believe they have stumbled across a network of secret Soviet tunnels beneath its foundations - and perhaps a nuclear bunker capable of holding 4000 people.

The discovery was reported during the demolition of the infamous Rossiya Hotel, built to accommodate Communist Party officials attending conferences in the nearby Kremlin.

An entire historic district of Moscow was levelled for the 3000-room hotel, which was the world's largest when it opened in 1967. But now the Rossiya is being demolished to make way for a re-creation of the Tsarist-era Zaryadye district, a US$800 million ($1.2 billion) project.

Shalva Chigirinsky, the millionaire developer whose firm won the building contract, said mysterious agents - presumably from the Federal Security Service (FSB) - were keeping his workers away from part of the hotel site.

"Some people in plain clothes are not allowing us access to an area where there is a tunnel from the Kremlin," he told the Komsomolskaya Pravda newspaper.

The authorities have not responded to his claim; indeed, the FSB has been known to call in Russian journalists for questioning, merely for speculating about what might be hidden beneath the streets of Moscow.

Since the collapse of the Soviet Union, however, there have been credible reports that subterranean Moscow is honeycombed with secret facilities whose construction began in the late 1940s, under Stalin. The network of tunnels apparently exposed by the Rossiya's demolition is believed to run from the Kremlin beneath the hotel and out beyond Moscow's Garden Ring road.

They are thought to be kept in working order, "just in case".

The tunnels, and a bunker also said to have been found by workers, are assumed to have been meant for Communist Party officials and the Soviet leadership in the event of a nuclear war.

"Diggers" - amateur adventurers who spend their spare time crawling through the city's sewers - have long claimed to have come across such tunnels. There have also been persistent rumours about the existence of a Moscow metro line which the public has never seen.

The line, called Metro-2, was apparently commissioned in 1967 to link the Kremlin and the headquarters of the Communist Party Central Committee with a huge 200ha underground bunker at Ramenki in Moscow's suburbs, and with Vnukovo-2 airport, 27km south-west of the capital.

The Ramenki underground complex, said to have been finished in the mid-1970s, was reportedly equipped to sustain the lives of thousands of people for up to 30 years. It had - or still has - food depots, generators, sleeping quarters, cinemas and even swimming pools.

The authorities have never confirmed the existence of such facilities beneath Moscow, but Russians, wedded to conspiracy theories, are convinced they exist.

If the Hotel Rossiya has been one of the many access points to this secret world for the past 40 years, it will soon be covered by concrete again.

In 2009, when the redevelopment is finished, the site will be a self-contained "town within a town", including hotels, offices, shops, restaurants, a concert hall and cinemas.

Any secret tunnels will have to make room for an underground car park for 2000 vehicles.