Heads have long argued that the formula for calculating education spending has cheated schools at the Welsh chalkface.
Now an independent coalition of the National Association of Head Teachers (NAHT) Cymru, the British Medical Association and Sustrans, a sustainable transport charity, are out to prove it Together, they are demanding an inquiry into the Barnett formula, which they claim is losing public services in Wales millions of pounds every year. And they have vowed to lobby the newly formed Assembly government after the May 3 elections to launch an inquiry on the scale of the devolution-making Richard Commission.
The Barnett formula, drawn up by Joel Barnett in the 1970s, works out money for public service based on population. Critics say the method should be reformed to take in need.
It has also been attacked for being incomprehensible, adding to the funding fog accusations that have dominated education spending in Wales.
Anna Brychan, director of the NAHT Cymru, said heads were traditionally concerned that the Barnett funding formula did not reflect true needs in Wales.
"Our members are seeking a fair and transparent funding system," she said.
The joint campaign was launched in Cardiff this week.
But the Assembly government has already indicated that there is no guarantee more money would be available even if the formula was replaced.