The heads suggest parents and the Government should share the cost of the places, on average between Pounds 400 and Pounds 500 a term. Their plan is modelled on the existing Assisted Places Scheme through which the Government pays for children from lower-income families to attend private schools.
The nursery proposal has been put before the Government's task force on nursery education and appears to have won the tacit support of education ministers.
Informal discussions have already been held between Robin Squire, the education junior minister, and members of the Incorporated Association of Preparatory Schools. John Morris, IAPS general secretary, said Mr Squire had been "very interested".
Ministers have been wrestling for months with strategies for fulfilling John Major's "cast-iron commitment" to provide nursery education for all four-year-olds whose parents want it.
It now looks as though a two-stage plan to increase the number of places and then give parents cash vouchers could form the basis of a summer White Paper.
Last month it was reported that ministers were close to agreement on a "bidding" system to be run by a quango or the Department for Education, which would create places, probably on a regional basis.
It was claimed that Government cash would then go to fund a voucher system once sufficient places had been created, although this could take several years.
The IAPS proposal could be attractive to ministers because the places already exist. Also, the independent sector has been expanding its pre-prep and nursery sector over the past decade. In 1984, about 8,000 pupils in IAPS schools were under five. This year the number had reached 15,800.
It is already possible for children to go to either a pre-prep or independent nursery and then on to a state primary school, but parents must bear the pre-school costs.
No figures have been set on specific levels of grant or whether it should be means tested. Nor has the IAPS drawn up specific proposals involving individual schools.
The Labour party has pledged to phase out the Assisted Places Scheme which provides day places for children aged 11 and over in nearly 300 schools.
John Morris said: "We have suggested to ministers a closer partnership between prep schools, Government and parents. We have had informal discussions with Robin Squire about this and other matters and he seemed very interested. "
A working party set up by the IAPS, Britain's largest association of independent schools, is to launch a year-long inquiry into the range and quality of nursery education in prep schools.
The inquiry will investigate issues of school size and age range, the quality of education experiences, length of the school day and integration of the nursery with the main prep school.
Robin Peverett, IAPS director of education, said: "The rapid expansion of pre-preparatory departments has led to a change in the overall character of the majority of British prep schools.
"We believe that provision for under-fives should be educational, that it should be staffed by trained teachers with proper ancillary support and that there should be ample space, good equipment and appropriate furniture and facilities. In our experience this is not cheap."
The working party will be chaired by Mary Head, headteacher of St Teresa's preparatory school in Effingham, Surrey.
It will make recommendations to the IAPS and its findings will be given to inspection teams which regularly visit IAPS prep schools.