Former Education Secretary Justine Greening says it is "wholly inappropriate" for school funding decisions to have become part of a legacy wrangle in the final days of Theresa May's premiership.
There have been reports that the prime minister is planning a significant increase in spending on schools in England before leaving Downing Street.
But Ms Greening says such a last-minute political intervention is "no way to deal with the question of long-term schools and education funding".
School funding is "far too important to be horse-traded like this", said the Conservative MP.
There have been long-running protests by school leaders over funding shortages - including a protest march by head teachers through Westminster and letters sent to millions of parents.
They have warned of staff cuts and having to reduce what schools can provide in terms of subjects and support.
The Institute for Fiscal Studies has said that schools have faced an 8% real-terms cut in per-pupil funding since 2010.
A response from the government was expected in the next public spending review - but ahead of that, Conservative leadership contender Boris Johnson has promised to boost school spending by about £5bn.
There are reports in the Financial Times suggesting Mrs May is planning to pre-empt this, with an announcement of extra school spending of up to £3bn.
The Times suggests that funding of £5bn could be made available.
There are also reports that the Treasury has resisted major spending commitments, which will stretch beyond the current prime minister's time in office and ahead of decisions about Brexit.
But a No 10 source urged caution about such speculation - and said nothing had been finalised.
Ms Greening, education secretary until the Cabinet reshuffle of January 2018, criticised the funding dispute as an "unedifying spectacle all round".
"School funding issues should have been recognised by No 10 and No 11 much earlier," she said.
There was also a question about how a one-off announcement would work for schools - when any rise in per-pupil spending would have to be sustained in subsequent years.
Such funding uncertainty would make it "very hard for teachers and schools to plan ahead", said Ms Greening.
Jules White, the West Sussex head teacher who has organised the Worth Less? funding campaign, said he shared Ms Greening's concerns about such "ad hoc school funding announcements".
He said the government has been in "abject denial about the state of school budgets" and had failed to act - and now seemed to be motivated by internal politics.
Mr White said schools needed an extra £5bn to reverse funding cuts - and not "sticking plasters".
A government spokesman said funding for schools in England was "at its highest ever level".
"But we know schools face budgeting challenges, and the education secretary has said he'll back head teachers with the resources they need."