Some reception staff in schools striving to boost test scores feel under pressure to work on formal literacy and numeracy skills, as well as encouraging play.
Already, strategies such as "catch-up" classes for five-year-olds have sparked accusations that these pupils are being prepared for key stage 1 tests.
Kathryn Relton, deputy head at Kings Road primary, Manchester, said:
"Writing is a priority here and it is very difficult to know if we should also be concentrating on it in the foundation stage."
The distinct teaching stage for children aged three to five was introduced in September. It is accompanied by early-learning goals, that set out what children should achieve by the end of reception year.
The guidance was welcomed by early-years practitioners and was seen as a means of resisting the pressure to introduce the literacy and numeracy hours before key stage 1.
Ms Relton said that, though staff liked and used the guidance, they were "concerned about to what extent the national curriculum should appear in reception class".
Staff in her school's nursery already use books that introduce children to the characters who appear in key stage 1 books.
Nansi Ellis, Association of Teachers and Lecturers primary adviser, said:
"Many people think that we are concentrating too much on 'sums and sentences' and not looking at the broader issues like personal and social development."
Fears that the testing culture is now affecting very young children have been fuelled by plans to move baseline assessment to the end of the reception year.
Currently the assessment is used to gauge what level pupils are at when they start reception.
The National Union of Teachers fears that an assessment at the end of reception would in effect become a test of how schools have taught the foundation stage.
The Qualifications and Curriculum Authority is about to make recommendations to ministers on the timing of baseline assessment.