The news that Ministry of Defence officials are trying to work out how to bomb the country without causing such a catastrophe came as B-52 aircraft arrived in Britain yesterday in preparation for the raids.
The Serbs' chemical- weapons capability was exposed three years ago in a World in Action documentary, "The Unseen Enemy". It is also believed Mr Milosevic has helped to build similar facilities for President Saddam Hussein in Iraq.
Although it is not known precisely how much of the chemical has been prepared in Serbia, it is clear 30 tonnes were transferred from an abandoned plant at Mostar three years ago. It was taken to Lucani, west of Cacak, where one of two remaining plants is sited. It can make 250kg a day.
The other plant, at Baric, near Belgrade, can make 120kg a day. Because it is in the midst of anti-aircraft defences, it could easily be hit during a raid on military installations. Makers of the documentary have helped officials to pinpoint the site, but fear a well-intentioned raid could end in disaster.
"You might easily break open the Sarin and release it to the atmosphere. Depending on the wind direction, you could get everybody in Belgrade. If that happened, Nato would be the bad boys of the world.
"It does concern me that there is this presumption you can do anything with fire-power," one said.
A Ministry of Defence source said it had made it clear any strikes would be against military targets. "By definition, if you are pursuing air strikes against military targets you want the best information to enable you to pursue that policy. I would not turn it round to say that we regard it as a matter of concern."