The Code of Practice for the identification and assessment of special educational needs is likely to be undermined by the rest of the education policies, according to Baroness Warnock.
Speaking at last week's Oldham National Special Educational Needs Conference, the former Mistress of Girton College, Cambridge, and a leading figure in the special needs debate, said that the new code was "incompatible" with recent legislation, and unworkable in large classes.
The code, introduced last autumn, sets out what schools and local education authorities must do to help vulnerable pupils. It has been welcomed by special needs campaigners as a mechanism for ensuring that children receive the support they need, and as a useful addition to the 1981 Education Act.
It was the recommendations of the Warnock committee which led to the 1981 Act and the current special needs framework.
"The code essentially depends on having a higher, not a lower number of teachers proportional to the children, a smaller not a higher number of children in the class," said Baroness Warnock, speaking to the conference on a pre-prepared video.
"Whether the child is dyslexic, whether he has a very short attention span, whatever the problem is, the idea of picking that out in a large machine like the teaching system seems to me to be actually almost impossible.
"I now see that the code lies very uneasily in the last education Act and indeed lies very uneasily within the 1988 Education Act and competition between schools. And also now the Government view of how teaching ought to be carried out. They seem to think it doesn't matter really whether you have 30 or 45 children in a class - you just teach them."