Liberal Democrats will scrap the married tax allowance to fund free childcare for every two-year-old in England if they are in government after next year's general election.

The plan is a central plank of a "pre-manifesto" of around 300 policies being unveiled by Deputy Prime Minister Nick Clegg under the slogan "Stronger Economy, Fairer Society", and is likely to be a sticking point for the party in any coalition negotiations if the election result is inconclusive.

The 80-page document will form the basis of the Lib Dem manifesto for the May 2015 poll, subject to approval by the party's annual conference in Glasgow next month. Activists will be able to amend the policies and, while their votes are not binding, it is unlikely the leadership would press ahead with any which are roundly rejected.

Lib Dems would guarantee to extend the 15 hours a week of free childcare, currently available to two-year-olds from disadvantaged backgrounds, to all two-year-olds by the end of the parliament in 2020 - with the ambition of achieving it within one or two years.

The plan to fund the bulk of the £800 million annual cost of the scheme by scrapping married couples' tax allowances could be a major obstacle in any coalition talks with Conservatives, who forced through the tax breaks despite opposition from their Lib Dem partners during this Parliament.

The allowance, worth up to £200 a year from next April to married couples where one spouse does not work, was described by a senior Lib Dem source as "a penalty on unmarried couples", including widows and abandoned wives.

And the party aims to go further, when resources allow, to extend the free childcare offer to all children aged nine to 24 months where both parents - or one in a single-parent household - are in work, before later increasing the maximum of hours funded by the state from 15 to 20.

These longer-term ambitions would cost an estimated £2 billion a year and the Lib Dem source acknowledged that the party has not yet identified a source of funding for them.

The source rejected suggestions that the party was penalising traditional families where mothers stay at home to look after children by withdrawing their tax breaks while failing to offer childcare support.

"If they have a two-year-old child, they are core beneficiaries," he said, adding that the party expected a take-up rate of around 85% for free childcare for two-year-olds.

Liberal Democrat business minister Jo Swinson said: "Helping families with the cost of childcare is part of the Liberal Democrat plan to build a stronger economy and a fairer society where there is opportunity for everyone to get on in life.

"Extending free early-years education to all two-year-olds will benefit children and reduce childcare costs for working parents. It builds on our record in Government where we have expanded free early-years education for every three and four-year-old and provided it for the first time to two-year-olds from the most hard-pressed families.

"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."

A Lib Dem source said that the key theme running through the manifesto will be "spreading opportunity through education", with a raft of policies aimed at helping younger people, parents and school-leavers

It will include a series of policies floated by Lib Dems over the summer, including protection for education spending; a parents' guarantee that children will be taught the core curriculum by qualified teachers; free school meals for all primary pupils; guaranteed lessons in sex and relationship education, financial literacy and citizenship; and a "Daddy month" extension of paternity leave.

The appeal to young people comes after the Lib Dems lost the support of many students by U-turning on their 2010 manifesto pledge not to raise university tuition fees.

Labour children's spokeswoman Lucy Powell said: "There's no point the Lib Dems making more promises when people remember only too well the promises Nick Clegg and his party have already broken.

"Voters know they cannot trust Nick Clegg and the Lib Dems. When it comes to family policy, the Lib Dems' record is trebling tuition fees, 628 fewer Sure Start children's centres and, by 2015, taking away up to £15 billion in support for children and families.

"Labour's plan for 25 hours free childcare for working parents of three and four-year-olds goes beyond the Lib Dems' bogus promises."