Plans to extend early-years education are at risk of going "pear-shaped" and downgrading teaching to a childminding service, the NUT has warned.
The extension of three and four-year-olds' entitlement to free early years education from 12.5 to 15 hours a week, which is to be introduced this year, will spread qualified teachers too thinly and undermine the quality of care given to pupils, the union said.
The extended hours will be available to the 25 per cent most deprived children from this September and to all children from September 2010. But an NUT survey of early-years staff has raised concerns that the initiative is not receiving adequate resources.
In areas that have already introduced extended hours, more than half of teachers reported an increase in working hours, with others reporting a reduction in planning time.
Almost seven out of 10 said the pattern of the school day had been changed, with later finishes to allow for the timetabling of more pupils.
In areas yet to introduce the initiative, more than half of the 259 respondents said there had still been no discussion of how the extended entitlement would work, despite it becoming mandatory in less than six months.
About two-thirds of teachers said they were concerned they would have to work longer hours and have insufficient time to track children's progress properly. There were further worries that increased use of unqualified teachers would damage the quality of education on offer.
"Children at this age need a rich environment with the guidance of a teacher," said John Bangs, the union's head of education. "Reducing that time is fatal."
He added: "This is an initiative that could go pear-shaped. This is a wake-up call for Ed Balls (Schools Secretary) and local authorities as well."
Sarah Tomlinson, assistant secretary of the NUT in Lambeth, told a fringe meeting at the union's annual conference in Cardiff this week that the extension of provision was "fraught with problems".
"The Government is increasing the entitlement without increasing the provision," she said. "Are we in danger of diluting nursery education and just providing a childminding service?"
Another speaker said the extension of early years was a move towards "childcare on the cheap".
The main conference unanimously passed a motion demanding that early-years education be protected by ensuring that it is delivered by qualified teachers.
Christine Blower, acting general secretary of the union, said in her closing speech: "The lack of funding or realistic plan for how this can be implemented has left many feeling that early-years teachers are being used by the Government to deliver its political commitments without the necessary support and investment. It is unacceptable."
The union also criticised the 300 "tick-box" targets being used to assess under-fives following the introduction of the early-years foundation stage framework last year.
A pilot project of the extended-hours initiative has been held in 20 local authorities since April 2007, with a further 14 councils joining in September last year.