It was probably the largest biological weapons programme the world has ever seen. And until the early 1990s, when the Soviet Union was falling apart and confidential information was already widely circulating, it remained one of its most tightly held military secrets.

As global attention is focusing on who could be behind the anthrax attacks that have hit the US in the past few weeks, Russia has been frequently cited as a possible source: if not for the direct manufacture of spores, at least for the expertise to do so.

The problem is few individuals have much knowledge of the Russian biological research and production complex. Most who do and are willing to talk about it are now out of date.

Kanatian Alibekov, the former deputy director of the Soviet bioweapons directorate who is also a specialist on anthrax, defected to the west in 1992. Now living in the US and known as Ken Alibek, he highlighted past abuses and a substantial continued threat in his 1999 book Biohazard.

Russia began developing biological weapons as early as the 1920s. Despite being a signatory to the 1972 Biological and Toxin Weapons Convention, designed to outlaw further bioweapons work a decade after the US and UK had already abandoned similar activity, Russia continued and accelerated its efforts.

Under the cover of Biopreparat, an organisation ostensibly set up to develop pharmaceuticals and vaccines for civilian use, it employed more than 40,000 people including 9,000 scientists in 47 facilities across the Soviet Union. Mr Alibek says it stockpiled anthrax, smallpox, plague, glanders and other agents.

The first sign of trouble came at Compound 19, a laboratory in the closed Urals city of Sverdlovsk, now known as Ekaterinburg, where former president Boris Yeltsin was once party boss. A leak of anthrax in 1979 led to widespread infection, triggering 64 deaths officially and possibly several hundred.

Mr Alibek alleges that former president Mikhail Gorbachev authorised a fresh programme in 1985, which included mounting "weaponised" germs and viruses on missiles aimed at the US. Only in 1992 did his successor, Mr Yeltsin, officially acknowledge Russia's illegal programme, ordering its closure and the destruction of stockpiles.

He says this order was implemented, stressing that it was not only difficult to stockpile bioweapons but also unnecessary. Instead they could be rapidly manufactured in the event of war.

The programme did bring advantages. In a country that suffers natural outbreaks of anthrax killing several dozen people a year, stocks of vaccines were manufactured. The former Soviet Union refused to give these to US troops in the build-up to the Gulf war in 1990, but they have been offered to them this month.

Mr Alibek suggests that, judging by the published scientific papers of some of his former colleagues, work related to the development of biological weapons continues in Russia today, including of genetically altered antibiotic-resistant strains of anthrax.

Biopreparat was only part of the bioweapons complex, with the Ministry of Agriculture and other government agencies employing up to 30,000 more people. Most important was the 15th directorate of the Ministry of Defence, in charge of "biological defence", created in 1992.

In an interview in the Russian Atomic Control Journal two years ago, Lt Gen Valentin Yevstigneyev, its then head, said it controlled the Kirov microbiology centre, the Ekaterinburg antibacterial defence centre and the Sergiyev Posad virology centre near Moscow.

He said the military held more than 100 different anthrax cultures in its collection, alongside samples of cholera, botulism and other diseases.

Ironically, the new administration of US President George W. Bush this summer renounced long-standing calls for the creation of such a mechanism for bio-weapons. It argued that mutual inspections of US biotechnology sites by foreign scientists could risk violating commercial secrets.

Professor Matthew Meselson from Harvard University, who worked extensively on the Russian biological programme, says he believes none of Russia's three Ministry of Defence centres have ever even been visited by foreign observers.

Amy Smithson, an academic at the US-based Henry Stimson Centre and author of The Toxic Archipelago about Russian biological and chemical weapons, warns that one risk is potential security breaches that could lead to substances being smuggled out or sold. Russian officials reject such suggestions.

But just as important is the danger that among the many thousands of scientists who worked on the programmes, many now very poorly paid or out of work, some may have been tempted to work for foreign countries or terrorists. There is little proof so far, but the size of Russia's programme generates continued fears.