A young, poorly-paid workforce with an unco-ordinated range of qualifications cannot teach an academic curriculum in the early years, says Professor Kathy Sylva, from Oxford University.
At a childcare conference in London, she said: "The Qualifications and Curriculum Authority knows that at key stages 1 and 2 the curriculum is taught by a trained workforce with degree-level qualifications.
"How can we expect an early-years workforce so varied in age experience and training to deliver such a subtle and balanced pedagogy in the early years?" She said it cannot be done on the basis of just one afternoon a week's in-service training.
A new study from the Early- Years National Training Organisation showed that 70 per cent of nannies and 44 per cent of nursery nurses in private day nurseries are under 24. Just under 30 per cent of nurses employed in the private sector have no formal qualifications compared to 15 per cent in school nurseries.
In at least 30 local authorities there is still no requirement for child minders to have preparation training.
Gill Haynes, chair of EYNTO, said the figures showed the sheer scale of investment needed in the sector. However, Mike Hipkins, manager of the early-years division of the Department for Education and Employment, said the private and voluntary sector had "a long tradition of combining education with wrap-around care".
Professor Sylva's warning came as the Government announced it had approved 150 local authority schemes for pre-school education.
Junior education minister Margaret Hodge said this means all four-year-olds will now be guaranteed a free nursery place.
The local schemes involve private and voluntary organisations as well as government bodies, bringing play groups, day-care centres and school nursery classes into a single structure.
None of the schemes was judged to have failed. But, while the childcare plans got full backing in 102 areas, a further 48 had only temporary approval. Councils including Portsmouth, Knowsley, Islington and Blackpool will have to resubmit their schemes in June.