Ministers have been urged to rethink plans to make all toddlers follow a so-called national curriculum from next autumn.
A panel of academics, including leading child psychologists, warn the government's forthcoming early years foundation stage will force the under-fives to follow an overly prescriptive curriculum.
The Department for Children, Schools and Families (DCFS) denies the early years foundation stage amounts to a national curriculum and says learning will be play-based.
In a letter sent to the Times, the academics warn the early years foundation stage could harm children's educational development and risks creating behavioural problems later in life.
They are also critical of the manner by which the early years foundation stage has been introduced, claiming it has been rolled out by stealth.
The scheme takes effect from September 2008 and will require all three- and four-year-olds to write simple sentences complete with punctuation, interpret phonic methods to read complex words and use maths to solve problems.
Launching the Open Eye campaign to oppose the early years foundation stage, the academics wrote: "An overly formal, academic and/or cognitively biased 'curriculum', however carefully camouflaged, distorts this learning experience.
"An early 'head start' in literacy is now known to precipitate unforeseen difficulties later on - sometime including unpredictable emotional and behavioural problems."
Today the DCSF hit back at claims the early years foundation stage is a "stealth curriculum".
A department spokesperson said: "This is a play-based approach to learning, development and care for young children which has been developed with thought, consultation and [the] wholehearted backing of early years specialists."
They added: "To suggest otherwise is scaremongering."
The Liberal Democrats nevertheless urged the government to reconsider its early years education strategy.
Children spokeswoman Annette Brooke said the government risks damaging a generation's education by pushing through prescriptive reforms.
She said: "There are serious concerns about the long-term effect of forcing children to learn how to read and write too early.
"Ministers must look again at their plans for early years education to ensure that children's chances, in this crucial time in their development, aren't wasted."