The key group pushing for a fairer funding system in England's schools says government plans for a funding shake-up need to be rethought.
The F40 group, which campaigned for 20 years for the funding overhaul, is warning that some poorly funded areas, and schools within them, will lose out.
Its vice-chair, Cheltenham MP Alex Chalk, says it is important "one injustice isn't replaced with another".
The government says it is not cutting funds in low income areas.
Instead it plans to end unequal funding across the country.
Education Secretary Justine Greening has highlighted how a school in Barnsley could receive 50% less than a school facing similar challenges in Hackney, east London.
This is because of extra funds having been directed in the past to areas of higher deprivation and need.
But many argue this is based on historical levels of need which are now out of date.
The cross-party F40 group represents 41 local authority members with historically low education funding.
Supported by MPs, councillors, education directors, heads, teachers and parents, it has long argued that the existing funding model is outdated, has no rationale and is unfair.
It has highlighted how pupils on different sides of a council boundary can attract very different levels of funding.
Ministers agreed the existing system was outdated and unjustifiable and, after a series of delays, the government published a final consultation on the proposals in December 2016.
The plans are set to be implemented in 2018-19.
F40 chairman Ivan Ould, a Leicestershire County Council member, said: "The proposed funding formula indicates a total gain of £183m for F40 member authorities once the national formula is fully implemented from 2019-20.
"But that has to be tempered by an outcome that none of us really anticipated: that some poorly funded authorities will not gain, and that many schools, both primary and secondary, within poorly funded authorities will actually lose out."
Earlier this week, the group lobbied MPs, many of whom are alarmed that so many schools in their areas are losers.
The group argues:
- Plans to limit budget cuts for losers to 3% will "lock in" historical advantages for schools "overfunded" for decades
- As the fair funding formula does not increase the overall schools budget, there could be severe problems for those who have to reduce budgets, especially in lower funded areas
- The new formula gives too much weight to additional needs as opposed to per pupil funding, and risks replacing one unfairness with another
- The evidence to support the detail of the proposals is weak and not specific enough
The Department for Education said school funding was at record levels but said the system for distributing it was unfair, opaque and outdated.
And it claims f40 authorities will actually gain £210m rather than than the £183m the group has alluded to.
A spokesman said: "We are going to end the historic postcode lottery in school funding and under the proposed national schools funding formula, more than half of England's schools will receive a cash boost.
"Funding every child fairly and according to their needs is at the heart of delivering the government's pledge to build a country that works for everyone.
"We recognise that schools are facing cost pressures, which is why we will continue to provide support to help them use their funding in cost effective ways, including improving the way they buy goods and services, so they get the best possible value."
A spokesman added that it wanted to hear from as many schools, governors, local authorities and parents as possible in its consultation which runs until 22 March.