Research shows that the new secondary curriculum is leading to less time being spent teaching subjects such as geography and history, MPs have been told.
The curriculum was only introduced in September for Year 7 pupils. But last week the Royal Geographical Society told the House of Commons schools committee that already there was evidence it had reduced teaching time in "some of the foundation subjects" - compulsory subjects that are outside the core of English, maths, science and ICT.
Rita Gardner, director of the society, told The TES she had based her comments on a survey of 200 secondaries carried out by the Geographical Association, which found that 70 per cent said the time allocated to geography had decreased.
Dr Gardner said she understood the situation was similar for history. "We, therefore, worry about the maintenance of a broad and balanced curriculum as we go forward, and as increasing pressures continue to be placed on it," she said.
Rebecca Sullivan, chief executive officer of the Historical Association, said: "There is some indication that there maybe a drop in the time spent teaching history because of the way some schools are adopting the new curriculum and going for a cross curricular approach. But I have no firm evidence yet."
Professor Gordon Stobart, of the Institute of Education in London, told the same hearing that there should have been more preparation for the new curriculum.
"There could, therefore, be an argument that we are moving into this too fast and we will adjust it as it goes," he said. "But adjusting something when everybody is doing it sows confusion."
Dr Gardner warned the committee that the humanities were suffering: "The key issue with the national curriculum is the core curriculum's major bias in favour of the sciences at the expense of the humanities and all that they bring as humanising subjects.
"We risk spending too much time on sciences, partly because there is a very strong science lobby."
The society believes a "double whammy" of signals is influencing heads and teachers who are allocating less time to geography.
First, there is less prescribed subject content in the new curriculum, although there is no suggestion that less time should be spent teaching it.
Second, Dr Gardner said, the Qualifications and Curriculum Authority has emphasised skills over subject content.
"Some of the messaging early on around the new curriculum from QCA could be read as subjects being less important, and could also be read as (schools) needing to give less time to certain foundation subjects," she said.
"But if young people don't understand the world in which they live or the historical context of where they are now, they are less skilled as citizens in making decisions about some of the things that really do affect their lives."
The society said the QCA and Ofsted should be ensuring that the new curriculum was not downgrading subject content.
A QCA spokesman said: "The national curriculum enshrines pupils' entitlement to a broad and balanced curriculum. It is structured around subject requirements, but this has never been a prescription for how it must be delivered. Schools have always had the flexibility to present the curriculum in a way that best meets the needs of their pupils."