North Devon MP and recently-sacked Defence Minister Nick Harvey has described one of his party's flagship policies in government as "cack-handed".
He claims the pupil premium - extra financial support aimed at the poorest school children - is actually widening the funding gap between the richest and poorest local education authorities.
His own county of Devon happens to be one the worst-resourced LEAs in the country.
His criticism hinges on the sole test the government uses to assess eligibility: free school meal claimants.
Mr Harvey says this is a "very clumsy, crude measure of pupils' income levels and poverty".
"There are many areas, particularly rural areas, which we know from all the other economic statistics are actually very poor, very low income areas where - perhaps rather surprisingly - the free school meal claim count is way below average," he said.
"So when the pupil premium is divied up these very poor areas aren't getting the pupil premium on the scale they'd been expecting.
"And actually, far from narrowing the gap between the poorer areas' school funding and the more wealthy areas' school funding, it's actually widening."
Emblazoned across the front of the Liberal Democrat manifesto at the last general election, the pupil premium is one of the policies the party's leadership is proudest of implementing in government.
Nick Clegg told us this week that it was one of just four things they were "prepared to die in a ditch for" during the coalition negotiations.
As Nick Harvey points out, all three main parties were promising some kind of pupil premium if they won the election; the devil - or rather the delivery - would all be in the detail.
Just last week the much-trumpeted policy received a body blow from the schools regulator Ofsted.
A survey of 262 school leaders found only one in 10 saying the pupil premium had significantly changed the way they supported pupils from disadvantaged backgrounds.
Not great for the government - or the Lib Dems just ahead of their conference.
But Ofsted was essentially laying the blame on headteachers and governors for not making proper use of the extra money they received.
Nick Harvey's criticism is levelled squarely at his former ministerial colleagues.
According to him, many heads and governors won't have the opportunity to use or misuse the money - because they won't be getting it in the first place.
School funding is a particularly sensitive topic in the rural South West and, above all, in Devon.
The formula the government currently uses to calculate basic school funding leaves Devon one of the lowest-funded authorities in the country.
So, pupils in Devon receive hundreds of pounds less than the national average every year.
Compared to the best-funded LEA that gap broadens to a couple of thousand pounds per pupil.
A long-running campaign to get this formula altered hit the buffers earlier this year when the Education Secretary confirmed that nothing will change until after the next election.
On the back of that disappointment, Nick Harvey is now suggesting the pupil premium - of all things - is now making this existing gap worse.
Devon County Council acknowledges that the free school meal test causes huge problems.
But a spokesman says it's impossible to say whether the pupil premium is widening the funding gap.
The Department for Education told me that this year the pupil premium has been extended to children who have been eligible for free school meals at any point over the last six years.
According to their spokesman, "this means that schools are now receiving the pupil premium for many of the children that Mr Harvey is concerned about".