Ontario will move ahead with a massive and costly program to offer full-day kindergarten for all four- and five-year-olds despite an unprecedented $25-billion deficit this year, Premier Dalton McGuinty said yesterday.

It’s “essential” the province invest in the $1.5-billion-a-year program to build a stronger workforce down the road, he said.

Moving forward with all-day kindergarten — billed as a North American first — may require cuts in other spending areas, McGuinty warned.

McGuinty’s staff couldn’t provide a breakdown of how much will be spent on all-day kindergarten over the next five years, but the government has set aside $500 million over two years to start it up.

About 35,000 kids will be able to enrol in full-day kindergarten next September, which McGuinty promised will expand to 50,000 kids in 2011, and to all eligible children in 2015 at an estimated cost of $1.5 billion a year.

Teachers will take the lead but also work with early educators in the classroom, said Education Minister Kathleen Wynne.

Class sizes will increase, but there will be enough funding for two adults for every 26 children, McGuinty said. The current provincial cap for primary classes is 20 to 23 students.

If parents want child care before 9 a.m. and after 3:30 p.m, they will have to pay a fee, and the kids will be supervised by an early childhood educator.