"I suggest these accusations be taken with a large dose of salt. It smacks of sour grapes. To say that primary schools are responsible is ridiculous and way off the mark", said David Hart, general secretary of the National Association of Head Teachers.
Last week the Pre-school Learning Alliance (PLA) published a report which claimed the future of playgroups is in danger because of dwindling attendances. The group blames the ever-increasing number of four-year-olds enrolling in reception classes as being directly responsible.
PLA chief executive Margaret Lochrie said: "We did not set out to attack schools. We are saying two things: First and foremost, 800 groups have already closed and another 1,200 face imminent closure.
"Second, and this is an absolute truth, we are losing four-year-olds to schools. I understand that heads see this an attack, but why, if it's the truth?"
The PLA describes the closures as a direct result of "unregulated" competition from the state sector - in particular, the decision by schools and local authorities to lower the age at which children are admitted to primary schools. Many more children now enter reception classes immediately after their fourth birthday.
Ms Lochrie said: "We're not blaming schools because they didn't create the situation, the nursery vouchers scheme did. And although the vouchers have gone, the admission arrangements for them haven't."
Until its recent closure, Jan Cherrett was the chair of Bewbush Poppets Pre-school group in Crawley, West Sussex. She said: "Until nursery vouchers we had a really good relationship with the local primary, but the scheme forced us into direct competition with one another . . . although it could never be a fair competition.
"If I thought entering school early was good for the children then I'd have no compunction about it. Pre-school groups have maximum staff:children ratios of 1:8 - the idea of 1:30 in a school is horrific for four-year-olds."
The PLA claims some schools have been guilty of "almost blackmail" and using aggressive tactics by guaranteeing places only to those parents who enrol their children early.
Ms Lochrie said: "I'm not saying all heads have adopted such policies. But those schools which have don't have a leg to stand on in complaining at us for having pointed it out."
David Hart dismissed any suggestions of foul play: "Schools are actively seeking to enrol four-year-olds for all sorts of good reasons, and yes, not least to ensure they stay on at the school when they reach compulsory school age. I can't imagine why anyone should be surprised by that.
"Schools simply haven't been able to recruit four-year-olds to this extent in the past. It's a bit rich to accuse us of bad practice," he said.
The PLA represents 19,000 parent-managed pre-school and playgroups nationwide. Its report will be part of a formal response to the Government's consultation on early years development plans.
Ms Lochrie said: "The Government is committed to expanding high-quality nursery education for four-year-olds. Losing such a large chunk of provision in closures must reduce the likelihood of delivering these aims. That's the bottom line. "