The number of primary teachers in England has fallen - leading some unions to claim that more classroom assistants are taking whole classes.
Teachers now have many more assistants
The number of qualified nursery and primary school teachers fell by 3,160 from 2002 to 2005, the figures showed.
But there were 26,150 more classroom assistants over the same period.
The government said it was "nonsense" that assistants were being used to replace teachers - it said there were simply fewer school-age children.
The National Union of Teachers was not party to the government's workforce agreement with the main unions.
From September 2005, all teachers were entitled to spend 10% of their working time on planning, preparation and assessment (PPA).
Critics of the plans feared that primary schools - which tend to have fewer staff - would find it particularly difficult to cover staff on PPA time.
The NUT's chief objection was that it feared precisely that the big growth in the number of assistants would be at the expense of fully qualified teachers.
General secretary Steve Sinnott agreed there were fewer children - but that presented an opportunity to cut class sizes.
"Some schools are using classroom assistants to child-mind rather than employing a teacher," he said.
"That is not acceptable."
The National Association of Head Teachers was a signatory of the agreement but subsequently voted to pull out, arguing that its members lacked the funding to implement the changes.
Its general secretary, Mick Brookes, said: "I find these statistics alarming."
He added: "Whilst I recognise that we have far more well-trained teaching assistants, children actually need teachers in front of them."
But the Department for Education and Skills dismissed as "utter nonsense" the idea that teaching assistants were replacing teachers.
"On the contrary, they are there to free up teachers' time to do what they do best - teach," a spokesman said.
The National Association of Schoolmasters Union of Women Teachers said it was "predictable" that the detractors of the national agreement would "rush to use any slight variation in teacher numbers to seek to undermine and misrepresent it".
General secretary Chris Keates said: "The variation is due to falling rolls. There is absolutely no evidence to demonstrate that there is widespread, inappropriate use of classroom assistants.
"The national agreement was never about using classroom assistants as 'teachers on the cheap' and I am sure that head teachers will be dismayed by the claims that they would seek to compromise standards by using such strategies."