Welsh pupils have almost pound;500 less spent on them than their counterparts in England, official figures confirmed this week.
The average per-pupil spend by Welsh authorities in 2008-09 was pound;5,192 - pound;496 less than over the border. And the average funding gap between the two countries has grown steadily since devolution - from 2 per cent in 1999 to 9.6 per cent.
The figures also confirm the findings of David Reynolds, of Plymouth University, in research commissioned by TES Cymru last year.
He revealed that there had been 6.4 per cent less investment in education since devolution than the average for the rest of the UK.
Although he also found there had been significant investment in nation- building - including the promotion of the Welsh language and sport - Professor Reynolds was accused of running down Wales.
He told TES Cymru he felt "saddened" by the latest figures.
"Unless the Assembly government finds at least pound;250 to pound;300 more per pupil, I fear the system will approach meltdown," he said.
The figures also reveal huge variations between the education budgets of Wales's 22 authorities. The gap between Ceredigion, the highest-spending authority, and the Vale of Glamorgan, the lowest, was at 25 per cent, or pound;1,338 per pupil.
Dr Philip Dixon, director of ATL Cymru, said: "These figures nail once and for all any attempt by the Assembly government to wriggle out of the fact that less is spent per pupil in Wales than England."
Last November, a snapshot poll of heads by TES Cymru revealed that lack of funding was a major concern, and the majority - 90 per cent - strongly agreed that underfunding was having a negative impact on pupils' education in Wales.
Jenny Randerson, the Liberal Democrat's education spokeswoman, said the blame for the growing funding gap lay with the government, whose education policy was "headed in the wrong direction".
A Welsh Local Government Association spokeswoman said councils consistently invested more in education than they received from government.
An Assembly government spokeswoman said policy priorities in England and Wales had diverged since devolution, along with school funding arrangements, making direct comparisons "increasingly difficult".
She added that Assembly government direct spending on education stood at pound;1.8 billion - an increase of 134 per cent since the National Assembly was created.