A former government chief scientific adviser says the Ministry of Agriculture was driven more by politics than science in the way it dealt with mad cow disease.

Professor Sir William Stewart, who is giving evidence to the BSE inquiry, accuses the ministry in a written statement of being concerned with "secrecy and political expediency".

The accusation by Sir William, who advised the Cabinet on science between 1990 and 1995, bears out earlier criticisms made by other former government experts.

Several have said officials at the Ministry of Agriculture, Fisheries and Food (MAFF) played down the risks that BSE could pose to human beings.

But they would probably answer that they were only obeying political instructions from ministers.

Douglas Hogg: Treading fine line

Ministers consistently said BSE posed no risk. But with hindsight, it is clear that politicians who say something is "absolutely" safe - that they have "complete confidence" in a product - are taking a massive political risk.

Yet these are the words used by successive ministers to try to reassure people they could eat British beef without a qualm.

The politicians concerned - ministers like John Gummer, William Waldegrave and Douglas Hogg - could say in their defence that they had to tread a fine line.

If they had cast the slightest doubt on the safety of beef, they might have caused panic. And they would have plunged a multi-million pound industry into chaos.

So they went on telling us that British beef was the best in the world.

Political factors

This strategy came horribly unstuck on 20 March 1996.

Health Secretary Stephen Dorrell told Parliament that scientists believed there was a probable link between BSE and its human equivalent, Creutzfeldt-Jakob Disease.

There was panic with the beef industry descending into chaos from which it has not yet emerged.

So Sir William is undoubtedly right in telling the inquiry that political factors played a large part in ministers' thinking.

And there was tremendous secrecy, too. MAFF gave little away about the fears that frequently preoccupied senior officials.

What did scientists say?

The inquiry will also want to consider the scientists' own advice.

Ministers always claimed they were acting on the best scientific advice available to them.

The announcement of an apparent link between BSE and CJD was not only horrifying. It astonished many people who had followed the epidemic closely.

Ministers were clearly over-optimistic about the possible significance of BSE, and this may have been due to political reasons.

But the real questions the inquiry will want to settle are simple.

Did officials give their ministers the best scientific advice available at every stage? And did politicians, as they claim, act on that advice?