Labour would replace the system that governs how Wales and the devolved nations are funded, First Minister Carwyn Jones has said.
Mr Jones said that Labour would scrap the Barnett formula, which governs how much the assembly is given by the UK government.
He said the formula could not be defended.
In response, senior Tory Alun Cairns said Labour wanted to "rip up" a recent fair funding agreement.
The Barnett formula dictates the level of public spending in Northern Ireland, Scotland and Wales, allocating funding based on population size and the powers devolved to them.
Cash is determined on whether the UK government increases or decreases funding for departments that cover areas that are devolved.
It is named after its inventor, the former Labour chief secretary to the treasury Joel Barnett, but has proved controversial and has been deemed to have underfunded Wales over the years.
Recently a new funding framework between the Welsh and UK governments, which does not replace Barnett, added new protections for Welsh funding.
When asked if Labour would scrap the Barnett formula, Mr Jones told BBC Wales: "That's what the manifesto says."
He said: "It says in the manifesto that there will be a new funding formula based on need. That means having a new formula to replace Barnett.
"Nobody can defend a situation where we have a funding formula that is over 38 years old by now, that was based on the way things were in the 70s.
"We need to have a funding formula that is fair to all the nations and regions of the UK."
The Welsh Labour manifesto says: "We need long-term reform of how the UK allocates public expenditure to ensure that it reflects the needs of different parts of our country and that no nation or region of the UK is unfairly disadvantaged."
Welsh Secretary Alun Cairns said: "Just six months ago the Welsh Labour Government signed a fiscal framework agreement to provide fair funding for public services in Wales for the long term, but now Corbyn wants to rip it up in favour of his uncosted and empty promises."
A UKIP spokesman said: "UKIP would replace the outdated Barnett formula with a fair funding formula based on need.
"We believe that, combined with a redirection of resources from the foreign aid budget, EU contributions and the scrapping of HS2, this proposal will make the majority of working people in Wales better off."
Plaid Cymru's Jonathan Edwards said: "This is yet another example of the 'Welsh Labour' myth.
"The Labour Party, led by Jeremy Corbyn, has a manifesto produced in London, for London, that makes no commitment to give Wales a fair funding formula. In fact the Labour Party has already said it is committed to the Barnett Formula."
The Welsh Liberal Democrats have been asked to comment.