Access to free childcare for three-and-four-year-olds in England is to double to 30 hours a week under measures announced in the Queen's Speech.
If passed, the childcare bill will grant the entitlement to families where all parents are working.
The changes aim to help 600,000 children a year from 2017.
However, nursery providers say the existing scheme is underfunded with the budget for free childcare falling 20% short of the cost of provision.
Neil Leitch, of the Pre-school Learning Alliance, said: "As a result, many are being forced to increase the cost of paid-for hours [those hours above and beyond the free entitlement], resulting in higher childcare costs for parents."
The group represents the owners of 14,000 childcare groups across England.
The plans had been outlined in the Conservatives' election manifesto and Mr Cameron said at the time that its £350m-a-year cost would be funded through reductions in tax relief on pension contributions.
Currently, all three-and-four-year-olds in England are entitled to 570 hours a year of free early education or childcare at nurseries, play and pre-school groups, Sure Start children's centres, or with childminders. Two-year-olds from the poorest families also qualify.
The 570 hours equate to 15 hours a week over a 38-week school year but can be spread over the year.
Mr Cameron had said this was an improvement on the "shocking" situation the Conservative-Liberal Democrat coalition inherited in 2010, "where couples were spending as much on childcare as one of them took home in earnings".
Previously "for many second earners, work didn't pay because the cost of childcare was so high", he had said.
Childcare funding around the UK
The measures announced in the Queen's Speech only apply to England because responsibility for childcare is devolved to national governments.
The Scottish government offers all three and four-year-olds 600 hours of funded nursery provision per year, while all those in Wales are entitled to 10 hours per week in term time.
In Northern Ireland, pre-school education is funded for two-and-a-half hours a day, five days a week during term-time.
New legislation will require councils to publish information about childcare in their area, and other services available that may benefit local families.
In the run-up to the election, Labour had proposed a "guarantee of access" to childcare between 08:00 and 18:00 for primary age children, while the Liberal Democrats had criticised the Conservative plans for "ignoring working families with the youngest children".
In February, a Lords committee highlighted the problem of underfunding in the sector, saying nurseries were struggling to survive in areas where parents could not afford extra hours.
It called on the next government to ensure that free places were delivered without any extra costs to parents.
Department for Education figures showed the average cost of providing free childcare places was £4.47 an hour, while the government paid £4.51 an hour for each three-and-four year old on the scheme.