The Liberal Democrats will increase funding for education from 'cradle to college', with a manifesto commitment which they claim will mean spending £5 billion a year more than the Tories and £2.5 billion more than Labour by 2020.

The plans would see the education budget for two to nineteen-year-olds in England rise to £55 billion by the end of the parliament, protecting spending per pupil even as the number of children enrolled in classes increases.

Lib Dem leader Nick Clegg said the party's election manifesto would have a commitment to spreading opportunity at its heart.

He said: 'The manifesto has one simple ambition and word at its heart: opportunity.

'It's a very old, liberal idea, the idea that everybody should be able to live out their life to the full regardless of the circumstances of their birth, regardless of the income of their parents, regardless of where they come from.

'Education is the great liberator of people's potential so we need to make sure we support the education system in the next parliament to give every boy and girl the chance to thrive.'

The Lib Dems view the policy as a measure which will attract wavering voters in seats where they face a Tory challenge.

Under the Lib Dem plans, the education budget - covering nurseries, schools and colleges - would be maintained in real terms until the structural deficit is eliminated in 2017/18.

As the number of children is increasing, that will involve a slight reduction in per pupil funding until the books have been balanced.

Extra money will be pumped into the system with above-inflation increases to the education budget in line with economic growth once the deficit has been dealt with, meaning the amount of cash per pupil will be protected in real terms across the five years of the parliament.

The commitment to guarantee education funding is on the front page of the party's manifesto, indicating it would be a red line in any coalition negotiations after the May 7 election.

Mr Clegg, who visited a primary school in Southwark, south London, ahead of the publication of his party's pitch to voters, said: 'At the heart of our manifesto will be a commitment to protect the funding for nurseries, schools and colleges, from nursery to 19, from cradle to college, and make sure there is enough money even as pupil numbers expand in the next parliament.

'None of the other Westminster parties can say the same, both the Conservative and Labour parties want to cut the amount of money going into schools and nurseries and colleges because their plans don't keep up with extra pupil numbers.'

He said the plan was 'absolutely affordable', adding: 'What it means in pounds and pence is that we will spend £5 billion more per year on nurseries, schools and colleges than the Conservatives will. We will spend £2.5 billion more per year on nurseries, schools and colleges than the Labour Party will.'

Mr Clegg acknowledged that the first years of the next parliament would require further austerity, including the education budget being protected in real terms rather than enjoying above-inflation increases.

'As we are balancing the books there is still belt-tightening which is required. By the end of the next parliament - which I think is what a lot of people want to know, they want to know what you are going to do over the next parliament - we will be spending the money that our nurseries, schools and colleges need even as pupil numbers increase in a way that no other Westminster party will.'

The number of pupils aged between two and 19 is expected to increase by around 460,000 by 2020, from 10.37 million to 10.83 million. The current budget is £49.2 billion and will increase to £55.3 billion by 2020 under the Lib Dem plan, some £2.5 billion more than would be the case if the money was simply protected in real terms.

The Lib Dems said the extra money was the equivalent to paying for 70,000 teachers and 10,000 learning support assistants as pupil numbers increase.

Mr Clegg, who will give further details of his approach to policy negotiations in any future coalition talks when he launches the party's manifesto, said the party would campaign 'very hard' on education issues in seats where they face a Tory challenge.

The leader's election tour has focused on seats where the biggest threat is from the Tories and a handful of constituencies the Lib Dems hope to snatch from their coalition partners.

'What we have discovered is that there are a lot of still quite undecided what I call decent, fair-minded, centre-ground, Conservative-inclined voters who really, really dislike the Conservative plan for very significant cuts to their nurseries and schools and colleges when it is explained to them this is what the Conservatives wish to do,' he said.

'We are not going to be shy coming forward - given that we have got this on the front page of the manifesto, we have got a plan to invest in education - pointing out to those Conservative-inclined voters that if they vote for a Conservative candidate they'll not only get the instability of a minority Conservative administration in hock to Nigel Farage, they will also get real cuts to the things that they really treasure locally.'