It was a key Conservative pledge during the General Election and will be fulfilled a year earlier than planned from September 2016.
Cameron will announce that working parents of three and four-year-olds in some parts of England will be entitled to 30 hours a week. Currently they are only getting 15 hours for free.
Ministers say that up to 600,000 families will eventually benefit from the move that is worth around £2,500 a year on top of the £2,500 they can already save from existing free child care offers.
Speaking ahead of the introduction tomorrow of the Childcare Bill, which will enshrine the measure in law, Cameron said: "My message is clear - this Government is on the side of working people, helping them get on and supporting them at every stage of life.
"That is exactly why we are pressing ahead with these reforms so that not a moment is lost in getting on with the task, going further than ever before to help with childcare costs."
In light of the news, however, many have argued that the pledge is "underfunded".
Neil Leitch, Chief Executive from the Pre-School Learning Alliance, which represents 14,000 private, voluntary and independent groups, warned that councils were already paying them insufficient hourly rates to provide the existing 15 hours of free childcare.
He told The Telegraph: "I think we are at breaking point with just the 15 hours. Extend that to 30 and you will see a different position altogether."