The Liberal Democrats say plans to scrap the married couples' tax allowance and use the savings to fund free childcare show the party has a "fundamental difference in values" to the Tories.
The party wants extend free pre-school education to all two-year-olds, building on the expansion in free nursery care for three and four-year-olds since 2010.
Business Minister Jo Swinson said the proposals would "benefit children and reduce childcare costs for working parents".
She also hit out at the Lib Dems' coalition partners, saying: "This is ultimately a fundamental difference in values. Liberal Democrats want to help all families with childcare support and nursery education right the way through from the end of parental leave to the start of school."
"The Tories are more bothered about helping only some couples through a married couples' tax break," she added.
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