His reservations came in the same week that the Education Secretary Gillian Shephard said, in an interview with The TES, that it was not the Government's preferred option.
The Government is under pressure from the right-wing Centre for Policy Studies (CPS) for a free-market solution to expanding pre-school services and it was at a CPS conference in London that Mr Forth warned: "For every parent with a voucher, there needs to be a school place somewhere and if there is too much of a mismatch then the virtues and joys of a voucher scheme will not be forthcoming."
But he said the Government was listening very carefully to the debate on nursery education, was consulting widely, and was aware of the complexity of the subject.
Mr Forth's comments are likely to be based on calculations on the pros and cons and practicalities of a voucher system now going on in the Department for Education.
His message no doubt pleased the critics of vouchers - in a minority at the conference - who said the CPS's recommendation to pin them at Pounds 700 per annum nowhere near covered the actual cost.
And they will also have been reassured to hear him emphasise how "scathing" early-years specialists were about the quality of some reception classes with 25 to 28 children.
Mr Forth stressed that Britain was "extraordinarily unusual" in its provision of full-time education from five years old. Most countries started at six, he said: "We are already ahead of the pack."
Everyone in politics appeared to agree on the benefits of nursery education, he said. And when everyone agreed, it made him "uneasy" because there was a danger the right questions were not being asked before the policy was formed. Was the need for pre-schooling educational or parental, he asked. Could you educate a four-year-old in a meaningful way for half-a-day or a full day? There were too many people prepared to accept loose definitions of education for the under-fives, he said.
He suggested - as former education secretary John Patten did last year - that some assessment of five-year-olds might be necessary as a baseline for value-added school league tables in the future. What effect would that policy have on under-fives education, he asked.
Should only fully qualified teachers educate these children? Some said Qualified Teacher Status was not strictly necessary because of the different sorts of qualifications which provided greater specialism for an age group with an "astonishing" variety of needs.
Later Dr Sheila Lawlor, deputy director of the CPS and the author of a recent pamphlet advocating vouchers, recommended a low flat-rate voucher worth Pounds 700. A low-cost voucher would provide a full-time place for a child at a playgroup and a half-time place at a nursery, she said. A high-cost voucher ran the risk of squeezing out the voluntary sector. And a means-tested sliding scale from Pounds 500 to Pounds 3,000 would effectively be a family tax for Government to spend.
But Margaret Lochrie, chief executive of the Pre-school Learning Alliance (formerly the Pre-school Playgroups Association) and Dr Albert Osborn, of Bristol University's School of Education, were critical of vouchers. They said capital money for buildings was needed in inner cities, rural areas and new housing estates where there was no under-fives provision.
Chris Woodhead, the Chief Inspector of Schools, said the evidence on the benefits of pre-school education was less than conclusive but there was no doubt that the extra stimuli they were exposed to helped them to settle in school as five-year-olds.