The National Institute for Medical Research (NIMR) looks innocuous enough. The tall, prewar brick building sits on a 50-acre site in the affluent North London suburb of Mill Hill. It is flanked by a primary school on one side, a pub on the other and playing fields behind. What the casual observer would not know, however, is that the latest strains of the lethal H5N1 avian flu virus are frequently flown in from around the world and stored in a squat little annexe behind the main building known simply as “Containment 4”.

The institute is one of at least eight establishments in the South East that handle some of the deadliest pathogens known to man or beast.

Experts say that most of these establishments have exemplary safety records, and that the Government requires and enforces some of the world’s toughest standards. But they also caution that absolute security is impossible to guarantee, largely because of the human factor.

Security at the institute NIMR is “as good as it can be but nothing is 100 per cent”, Simon Caidan, the institute’s safety and security manager, admitted as he showed The Timesthe formidable system of filters, airlocks, negative air pressure, alarms and other technologies that prevent the avian flu viruses escaping from Containment 4.

“One can put in all the engineering and control methods in the world, but at the end of the day you still have to let human beings in and out,” said Anton de Paiva, biological safety officer at Imperial College, London.

The Health and Safety Executive (HSE) has authorised five establishments to work on Category 4 pathogens — biological agents that cause “severe disease in humans” and for which “there is no effective prophylaxis, or treatment, available”. There are 16 Category 4 viruses, including Lassa fever and Ebola.

The HSE will not name the establishments for security reasons, but they include the Health Protection Agency Centre for Infections at Colin-dale, northwest London, the Centre for Emergency Preparedness and Response at Porton Down, Wiltshire, the military Defence Science and Technology Laboratory at Porton Down, and the National Institute for Biological Standards and Control in South Mimms, Hertfordshire.

Approximately 450 other establishments - mostly universities and pharmaceutical companies - are authorised to work on Category 3 pathogens, biological agents that are dangerous to humans but for which treatments do exist. They include forms of salmo-nella, hepatitis and encephalitis, Mobala virus, yellow fever, Sars, BSE, HIV and Creutzfeldt-Jakob disease.

The Department for Environment, Food and Rural Affairs (Defra) licenses laboratories to work with some of the 13 viruses considered most deadly to animals. These include foot-and-mouth, rabies, avian flu and swine vesicular disease. Defra also refuses to name the establishments, but among them are the NIMR, the Institute for Animal Health (IAH) laboratory at Pirbright, and the Veterinary Laboratories Agency in Weybridge, Surrey.

Almost all these establishments have excellent safety records. A scientist at Porton Down contracted (and survived) Marburg disease, but that was back in 1976. Two laboratory workers, one at Birmingham University and the other at the London School of Hygiene and Tropical Medicine, were infected with smallpox in the 1970s.

The possible exceptions are the two IAH laboratories at Pirbright and at Compton in Berkshire. Pirbright was linked with a foot-and-mouth outbreak near Guildford in 1960, and Pirbright and Compton have between them received half a dozen “notices” from the HSE over the past four years. The most serious came last February when Compton was told to stop work on cattle infected with bovine TB because of a faulty ventilation system.

But Mr Caidan, the NIMR safety manager, said he was very surprised by the latest apparent leak of the foot-and-mouth virus from the Pirbright complex, which is governed by the same draconian security rules and regulations as his own. As Containment 4 is presently being refurbished, he agreed to show The Times around.

The NIMR includes one of four world influenza centres, and regularly receives new strains of avian flu, mostly from the Middle East and states of the former Soviet Union. Mr Caidan explained that only six scientists and three animal technicians are authorised to enter Containment 4. All will have had security clearances, and all will have received months of training — with exams. The laboratory is rigorously inspected each year by the HSE and Defra, and regularly visited by counter-terrorism officers.

Entry is through a steel door, using a swipe card and a code. Air pressure in the lobby is kept slightly higher than outside to keep out dirt and insects. Before entering the laboratory the scientists undress, put on lab clothes and pass through an airlock.

Inside the laboratory the air pressure is kept extremely low so particles are sucked in, not out — so low in the animal holding area that paint would be pulled off the walls. The viruses arrive in sealed plastic containers, are placed inside sealed biological safety cabinets, and the scientists can work with them only by putting their hands into holes in the cabinets’ glass fronts that lead into thick gloves. There is no physical contact with virus.

When they finish work, samples are killed with disinfectant, or stored in locked freezers. The scientists shower thoroughly before returning to the “clean side”. Their lab clothes and all other solid material from the “dirty side” are sterilised in an autoclave at 130C (266F). Waste liquids are passed through an effluent sterilisation tank heated to 137C. All air leaving the laboratory passes through filters that remove 99.97 per cent of particles. The laboratory is monitored by 20 internal cameras and nearly 2,000 alarms that are watched continuously.

The one thing Mr Caidan cannot control, however, is human behaviour. “There’s nothing to stop anybody putting their hands in the fridge, concealing something about their person and walking off with it. It would not be easy, but it’s possible,” he conceded.

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