Early-years experts, including private providers, immediately attacked the decision as a watering-down of standards and a green light for "cowboys".
A Department for Education and Employment welcome pack for nursery education providers registered to accept vouchers says pre-schools must provide a minimum of two 212-hour sessions before the first inspection by the Office for Standards in Education.
After the first inspection, which is planned to take place within a year of receiving vouchers, the pre-school must provide three sessions, and five sessions in the second year.
Dr Gillian Pugh, director of the early childhood unit at the National Children's Bureau, and chair of the Early Childhood Education Forum, which met last week, said: "The decision opens up the doors to cowboys."
Susan Hay, managing director of five private London nurseries, said: "We are concerned that the scheme is planned around the lowest common denominator. If they start this way, they will end this way. They won't make it any harder. "
She said parents should expect more than five hours a week nursery education from a Government voucher scheme.
It is understood that a minimum as low as five hours has been agreed because a large number of Norfolk pre-schools at present offer only two sessions a week. Norfolk is one of four education authorities taking part in the first phase of the voucher scheme from April.
But the Pre-school Learning Alliance defended the five-hour "concession" as "reasonable."
Margaret Lochrie, the alliance's chief executive officer, said: "It does reflect the fact that in Norfolk a number of groups are only able to offer two sessions a week. These are groups operating in very rural areas. There might be limitations on their premises. There are likely to be a complex of factors which will take some time to work through if children are going to be offered more than two sessions."
Norfolk County Council has told heads that playgroups offering only two or three mornings a week will have to provide at least three sessions within six months, and playgroups offering fewer than five sessions a week will have to offer at least five sessions, either alone or in collaboration, after two years.
The voucher, says the county, will have a proportional value according to the number of sessions offered - hence for three sessions the proportion will be three-fifths of the Pounds 1,100 voucher.
The DFEE said the voucher scheme could not be hijacked by cowboys because only providers registered under the Children Act were able to take part.
The forum meeting, which was attended by professionals representing about 30 under-fives organisations, is also angry about the arrangements for pre-school inspections.
Forum members are concerned that a total of 4,000 early-years inspectors are needed for the voucher scheme, yet most inspectors who are appropriately qualified are already tied up with clearing the backlog of primary inspections.
But an OFSTED spokeswoman did not anticipate any problems, saying only 300 inspectors were needed for the voucher scheme initially. The early years inspectors would be new ones from organisations such as the Pre-school Learning Alliance, which already had an accreditation scheme for its pre-schools.
OFSTED would be talking to umbrella under-fives organisations about the details of the inspection process, she said.