The Liberal Democrats have accused Michael Gove's department of "lying" and "talking bollocks" as a furious turf war erupted within the coalition government.
At stake is how Clegg's flagship promise of free school meals for every five to seven year-old.
The measure is due to be confirmed in George Osborne's Autumn Statement on Thursday.
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Clegg has announced an extra £150 million for schools to help them get their kitchens and dining rooms up to scratch so they can meet his free dinner plan.
But while he made a visit to a school in Lambeth as he prepared to confirm the cash, sources linked to the Department for Education are understood to have briefed reporters that the Deputy Prime Minister was possibly planning to raid an extra places fund to pay for the move - then dismissed claims unspent cash was available to cover the cost.
Clegg and Gove don't seem to get on
In a brutal counter-briefing senior Liberal Democrat sources accused the DfE of lying, going rogue, being hostile and "talking bollocks".
Downing Street then made clear that it was Tory Gove's department that was out of line.
"The position is absolutely the one the DPM's office have set out," the Prime Minister's official spokesman said.
The free school meals plan has also not gone down well in Clegg's own party, with questions asked as to why taxpayers are subsiding wealthy children's lunches at a time of austerity and sweeping cuts elsewhere.
Last month, former defence minister Nick Harvey told HuffPost UK it was "squandering money" and described his "open-mouthed astonishment" when it was announced.
Relations between Clegg and Gove have appeared strained for many months, with the Liberal Democrat leader criticising the use of unqualified teachers and blocking plans to allow nurseries and childminders in England to look after more children.
Today's announcement of new funding to allow schools to build kitchens and expand dining rooms will be included in Thursday's Autumn Statement. The Treasury will be stumping up £70 million and the remaining £80m will come from underspend in the school maintenance budget.
The money is on top of £450m set aside in 2014/15 and £635m in 2015/16 to fund the free lunches plan, which is expected to save parents around £437 per year, per child.
A senior Liberal Democrat source said: "This is a decision taken jointly as a Coalition government at the very highest level of the Coalition government at the Quad (Prime Minister, the Deputy Prime Minister, the Chancellor and the Chief Secretary to the Treasury).
"It was something that the Chancellor and Prime Minister entirely signed up to and that's why we have agreed the funding as we have.
"I think what you have seen from DfE is them going rogue and a slight bitterness at the fact that this policy wasn't invented there but they are completely out of step with the rest of government whether they are coloured yellow or blue and they need to come in line with the rest of the Government."
He added: "It's time for them to stop whining and get behind a policy that they are going to have to deliver because all of the rest of government is agreed with it."
Told that departmental sources were insistent that the £80m did not exist, he replied: "The DfE are talking bollocks. The Treasury and the DfE know, and David Laws is the one that has worked all this through, there is a £157m maintenance underspend of which we are going to spend £80m on this and they are going to spend the other money on free schools.
"That is a fact. The DfE are lying to you if they are saying otherwise."
Clegg said the government was "making the sums add up" to fund the schools meal plan by finding money from a number of sources.
At Walnut Tree Walk Primary School in Lambeth the Lib Dem leader sat with pupils to eat a school lunch of chicken and rice followed by crumble with custard, and said free meals would be "a great, great way of actually helping children concentrate in the afternoon after the lunchtime."