text: The prime minister said Labour had a "brass neck" to claim they were on the side of ordinary working families.
  Under the Â£350m Conservative plans, the amount of state-subsidised childcare for three and four year-olds would be doubled to 30 hours a week.
  Labour said it was "another unfunded announcement", accusing the Conservatives of "desperation".
  The Liberal Democrats said the plans "ignore working families with the youngest children".
  BBC political correspondent Carole Walker said Mr Cameron was attempting to switch the focus of the campaign to one of his party's key announcement after several days dominated by warnings about the SNP and a future Labour government.
  The party is likely to face questions about how it would ensure sufficient childcare places are available, she added.
  Currently, all three and four-year olds in England are entitled to 570 hours of free early education or childcare a year, which works out as 15 hours each week for 38 weeks of the year.
  The Conservatives said the 30-hour offer from 2017, announced a week ago, would result in more than 600,000 extra 15-hour free childcare places every year.
  They said the proposal would be funded by reducing tax relief on pension contributions.
  Labour has pledged 25 hours of free childcare a week and the Liberal Democrats 20 hours, although both parties also plan to extend the offer to younger children.
  Mr Cameron said his government inherited a "shocking" situation, "where couples were spending as much on childcare as one of them took home in earnings".
  He added that "for many second earners, work didn't pay because the cost of childcare was so high".
  A Conservative government would expand on the changes made in the last Parliament, Mr Cameron said.
  "If you're a working parent with one child you can rest assured that by the time they're three they'll be able to go to nursery for 30 hours a week completely free," Mr Cameron said.
  "And we have legislated also for tax free childcare for anything outside that - so if you spend ten thousand pounds on childcare you'll get two thousand pounds back for each child."
  But Labour's shadow education secretary Tristram Hunt said: "Hard-working families will not be fooled by the Â£600m gap in funding for this policy, as announced last week."
  He said Labour had a "better plan" including a "guarantee of access" to childcare between 08:00 and 18:00 for primary age children.
  And Liberal Democrat equalities minister Jo Swinson said: "Tory plans for childcare ignore working families with the youngest children, which could leave some parents locked out of the labour market for years on end."
  She said her party would extend early-years education to all two-year-olds and ensure free childcare support kicked in as soon as paid parental leave ended for working mums and dads.
  "This will help with the cost of childcare and ensure working parents have a genuine choice about when to return to work," she added.
  The UK Independence Party says it will continue to fund the current free 15-hour a week childcare scheme and in-coming tax-free childcare scheme - although they would de-regulate childcare provision "to address the shortage places and cut the cost to both parents and the state".
  * Subscribe to the BBC Election 2015 newsletter to get a round-up of the day's campaign news sent to your inbox every weekday afternoon.
target: David Cameron says he will create 600,000 extra free childcare places if he is returned to power next month.