If a single moment defined the 14-year crisis of "mad cow" disease it occurred on 8 March 1996 in a dismal government building in south London. The lights had been lowered for a slide presentation to be given to the Spongiform Encephalopathy Advisory Committee (Seac) by a young Edinburgh pathologist who had some remarkable pictures of diseased brains in his suitcase.

If a single moment defined the 14-year crisis of "mad cow" disease it occurred on 8 March 1996 in a dismal government building in south London. The lights had been lowered for a slide presentation to be given to the Spongiform Encephalopathy Advisory Committee (Seac) by a young Edinburgh pathologist who had some remarkable pictures of diseased brains in his suitcase.

James Ironside, of the National CJD (Creutzfeldt-Jakob disease) Surveillance Centre, which was set up in 1990 specifically designed to identify unusual disturbances in the pattern of classical CJD, was the first person in the world to see what bovine spongiform encephalopathy (BSE) does to the human brain. When he started showing his slides, the mood of the Seac members changed from business-like joviality to one of stunned silence. "There was an atmosphere of surprise. It was quite a significant occasion," Dr Ironside later recalled with some understatement.

John Pattison, the urbane chairman of Seac, remembered the occasion well. "When he showed us the slides and before he said anything, we could see what it was. It was dramatically different from classical CJD," Professor Pattison said.

With the vividness that only pictures possess, Dr Ironside had demonstrated to the assembled experts that a new brain disease of young people was stalking the land. It meant that after years of official reassurance it was now becoming clear that beef was not, after all, safe.

Dr Ironside began this particular picture collection in September 1995 as he peered down his microscope at Western General Hospital in Edinburgh. The slices of brain material had come from a teenager who had died of what appeared to be CJD, which usually affects people over 50.

It became immediately obvious that the teenager's brain had suffered highly unusual damage. Large, flower-shaped clusters, known as "florid plaques", had formed between the nerve cells. Dr Ironside had studied hundreds of diseased brains and had never seen one quite like this. By the time of the meeting six months later, he was to have seen seven more.

The last few months of 1995 was an extraordinarily anxious time for those involved in BSE. Several farmers had died of CJD and the media were asking awkward questions about whether their deaths were due to close contact with BSE-infected cattle. On top of this, it was becoming embarrassingly obvious that safety measures in slaughterhouses designed to protect the human food chain against BSE were being flouted.

Soon after BSE was identified in cattle in November 1986, there were concerns about whether it could be transmitted to humans. A number of safety measures were introduced in 1989, including a ban on bovine brain, spinal cord of other offal material known to be most at risk of carrying the BSE agent. Yet more than five years later, in November 1995, Seac was to learn that slaughterhouses were routinely ignoring the legal requirement to remove and dispose of so-called specified bovine offal (SBO).

Spot checks on 16 slaughterhouses by inspectors from the Ministry of Agriculture, Fisheries and Food (Maff) had found that a total of 30 pieces of spinal cord remained on dressed carcasses ready for delivery to butchers. Statisticians had estimated that about 0.4 per cent of carcasses consumed in Britain at that time might be contaminated - five years after measures had been introduced specifically to stop this from happening.

Robert Will, head of the CJD surveillance unit and then deputy chairman of Seac, was furious when the Maff informed him of the breach. The minutes of that Seac meeting in November 1995 testify to the anger that he and others scientific advisers felt after years of being reassured by the ministry that SBO material was being removed from carcasses. "I was appalled," said Dr Will in evidence to the BSE inquiry, which opened in March 1998. "The implication was that high [amounts of] tissue, specifically bovine spinal cord, might be entering the human food chain."

But Dr Will had another problem. As the head of the CJD surveillance unit, he had to decide what to make of the newly emerging cases among young people. As one of the most influential scientists in this field, he knew that much would hang on precisely what he said publicly about them.

By January 1996 Dr Will knew of five young people who had died of CJD, but he was still unsure as to whether this was simply the result of a greater effort being put into looking for CJD - called "ascertainment bias" - or whether it was a genuinely new phenomenon.

"I had become increasingly concerned about the young cases of CJD, but there was insufficient scientific evidence to reach a conclusion about the novelty of these cases, nor to reach a judgement about whether these cases might be causally linked to BSE," Dr Will told the inquiry.

Meanwhile, the media coverage of BSE at the end of 1995 had reached fever pitch. Sir Bernard Tomlinson, the eminent neuropathologist, started the ball rolling by contradicting the smug reassurances of ministers that beef was safe. The Oxford brain researcher Colin Blakemore also came out against beef, saying "the taste is just not worth the risk".

The Meat and Livestock Commission had launched a vigorous propaganda offensive in reply to the growing doubts over the safety of beef. One advertisement even quoted Dr Will, to the scientist's dismay. "I was unhappy when I saw the advert published because I did not think that it accurately reflected my views in that it linked my name to each of the facts/fiction statements in the body of the advert and I had not been shown a copy of the final version of the advert before it was published," Dr Will said.

As the first weeks of 1996 passed by, Dr Will's natural scepticism over what could be the cause of CJD among the youngsters came under intense strain. He gave a talk on 20 February 1996 to the Parliamentary Food and Health Forum, where he continued to make out a case for caution in terms of a possible link with BSE.

"The reason for this is that I did not on 20 February 1996 believe there was sufficient scientific evidence to make a judgement about whether these cases were truly novel, nor did I believe that there was sufficient scientific evidence to reach a judgement about a causal link between these cases and BSE," he said.

"It would have been completely irresponsible to voice my concerns at this meeting because this would have been likely to precipitate a crisis at a time when there was insufficient scientific evidence to make a judgement," he added.

"With hindsight and with the benefit of accumulated scientific evidence it is easy to forget that there was a real possibility in February 1996 that the young cases were not actually new and were not linked to BSE. If a crisis had been precipitated by speculation... this would have had potentially profound socio-economic and indirect public health implications."

Yet within weeks of this public meeting, Dr Will and Dr Ironside found themselves in the same room at Skipton House, a Department of Health building at the Elephant and Castle. They were there to advise Seac that the eight new cases of CJD among young people were most probably new to science and were very likely to be caused by BSE.

They said they would not release details until the work was formally published by a medical journal but that the final decision would be Seac's. Professor Pattison gave them another week to make further enquiries to see if the symptoms of the "new variant CJD" (v-CJD) could be found anywhere else in the world and arranged a further meeting of Seac in a week's time.

The following Saturday, on 16 March, Seac met again and asked Dr Will if any of his colleagues overseas had come across a similar set of symptoms; they had not. A ninth victim was also added to the list. Professor Pattison knew it was time to act. That Saturday evening, he informed the Chief Medical Officer, Sir Kenneth Calman, and the Chief Veterinary Officer, Keith Meldrum, of the findings.

Ministers were given the news on Monday. Douglas Hogg, then Minister of Agriculture, and Stephen Dorrell, Secretary of State for Health, told Michael Heseltine, the Deputy Prime Minister, that day. Professor Pattison briefed a meeting of the full cabinet on Wednesday morning, 20 March, along with the news that a 10th victim had been added to the list. The story had by now been leaked to the media and a press briefing was arranged along with a ministerial statement to the House of Commons.

It was left to Mr Dorrell to stand up and tell a packed House that the most likely cause of a deadly new brain disease in humans was BSE. It marked the point when all those bland government reassurances over the past safety of British beef finally crumpled in a wave of public outrage. No longer would politicians blithely use the word "safe" when it comes to human health.