Liberal Democrat education spokesman Phil Willis made the claims as he set out an alternative vision for schools which would scrap national curriculum testing and league tables.
Mr Willis told a fringe meeting at the party's annual conference in Brighton: "Children aged five or five-and-a-half are having to be referred to the doctor because of stress as they prepare for the national curriculum tests. We have to break out of the straitjacket of testing."
Later, unveiling the party's new education proposals to the conference, Mr Willis said: "It isironic that just as we are understanding the importance of free-range farming, we are turning too many of our nursery schools into little more than cages of battery hens.
"Children get measured amounts of approved learning that bear little relation to their needs."
The stress allegations come after several teachers posted messages to the TES website warning of the stress pupils were experiencing around key stage 1 tests, taken at the age of seven.
Many of the Lib Dems' proposals have already become policy in Wales. Since last year, achievement at key stage 1 in Wales has been measured solely by teacher assessment. The Welsh Assembly is also planning to introduce a foundation stage that postpones formal learning until the age of seven. But at its party conference last week, Plaid Cymru's education spokesperson suggested that children as young as six months would benefit from a national curriculum, including the studying of Welsh heritage.
Addressing the conference in Llandudno, Helen Mary Jones said that she would introduce a foundation curriculum for children from six months to seven years.
The content of the childcare curriculum would not be prescribed, but children would be expected to acquire skills such as learning how to use the toilet and blowing their noses.
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