Last night the report, published by Itar-Tass, was unconfirmed. If true, it will be the first time the Russians have admitted that weapons-grade fissile materials have been smuggled out of the country. It will also send a chill through every western security agency; for it is the scenario that they fear the most.
The story broke as both Moscow and London were yesterday keeping mum about the precise nature of Russia's spying accusations, but theories were coming as thick and fast as the traffic fumes that fill Moscow's streets. Britain and the West are as curious as ever about the contents of Russia's more secret closets.
After the collapse of the Soviet Union and the advent of "glasnost", western spying in Russia is believed to have increased, as agencies vied with one another to find out more about previously secret installations, people, and projects that they had been puzzling over for years. Much of that reservoir of information has been exhausted, but there are plenty of others.
The Russians say the alleged British agent whom they are holding worked in a federal government office with access to "political and defence and strategic importance to British intelligence" - a vague term covering a multitude of sins.
Arms control treaties and spy satellites have made spying largely redundant for gathering straightforward military intelligence. East and West can and do conduct detailed inspections of each other's installations, and are punctilious in honouring the provisions of the treaties. The Russians - and their western counterparts - have all bent over backwards to show each other as much of their military hardware as possible and also to talk about military organisation and doctrine.
It is therefore in the political and economic arena that cloak-and-dagger spying still has a role. With an election looming, for instance, the Foreign Office will have an appetite for political information, including the political and financial consequences of a Communist victory.
During the Cold War, the West devoted great attention to the Soviet economy - and assessing the resources it was pouring into the ability to wage a protracted war. The collapse of the Soviet economy has made that less crucial, although the exact strength of the Russian economy in certain areas remains of interest.
One such area is that of revolutionary new technologies, where Russia, for all its economic problems, maintains islands of excellence.Deep-diving submarines like the modified Alfa class, beam weapons, and extremely high and fast-flying aircraft are areas in which Russia still surprises western observers.
The most sensitive area now is the fate of nuclear weapons and materials, and arms exports. In particular, it would be surprising if the CIA, MI6 and the Israelis were not taking a strong interest in Russia's relationship with Iran, to whom it has sold several billiondollars' worth of weapons systems and diesel-electric submarines in the last few years.
They will be watching closely the nuclear power plant that the Iranians are building at Bushehr, 470 miles south of Tehran, using Russian-supplied reactors. Nor did they deny their curiosity about an underground complex the Russians have been building in the Ural mountains, under the gaze of western spy satellites. The project, hidden inside Yamantau mountain in Beloretsk, involved the creation of a huge complex, served by a railroad, a major road, and thousands of workers. Fears were raised that Russia planned to manufacture chemical and biological weapons.
Following a period of unprecedented openness between East and West, there are many signs that Russia may, once again, be closing its doors. On 9 March a decree by President Yeltsin demanded the "improvement of the system of State secrets" Mikhail Barsukov, the head of the FSB - the Russian security service, which formulated the accusations against the man accused of passing information to the British and his alleged controllers - said not enough things were classified as state secrets.
In the past year, several British industrialists, academics and business people have been accused of espionage, while attempting to assess what help the Russians might need. "I was accused of being a Nato spy", said one man yesterday "I told them that so far I had seen nothing worth spying on."