Headteachers are acting like "ostriches" by failing to prepare for "severe budget changes", the government official in charge of school resources has warned.
Many schools will suffer serious consequences unless they start to regard financial planning as part of the "day job", according to Dugald Sandeman, director of resources at the Department for Children, Schools and Families.
Mr Sandeman said a DCSF survey had uncovered a worrying attitude to whether schools were "ready and willing to cope with severe budget changes".
"All across the country there are people who are doing an ostrich act on this, which will be very, very detrimental indeed to their institutions," Mr Sandeman told a meeting of academy heads in London organised by the Independent Academies Association.
"The key issue is the cultural resistance to believing this is a critically important thing to do."
Schools have become used to planning year on year rather than taking a longer-term view, Mr Sandeman said.
"The challenge is particularly great for primaries because most of them do not have the capacity to deal with this," he said.
Mr Sandeman told delegates that a survey of schools and local authorities had given the DCSF "the quite strong feeling from a very large number of people" that "this is not what our day job is about".
"We are here for teaching and learning. Messing around with resources is a secondary thing." He said this approach needed to change or schools would be "lost" when tighter budgets kick in.
The criticism follows a conference run by the National College for Leadership of Schools and Children's Services at which more than 100 heads, governors and local authority officials were urged to use resources more effectively.
It was called at the request of the DCSF after Ed Balls, the Schools Secretary, told schools of the importance of making "efficiency savings".
The Chancellor has promised that spending on schools will increase by 0.7 per cent in real terms between 2011 and 2013.
This is a significantly reduced rate of growth compared to previous years.
Mr Sandeman said that decisions schools take from this September will be affected by the squeezed budgets, which will be introduced in April 2011.
Mick Brookes, general secretary of the National Association of Head Teachers, said schools have become used to funding growth.
"There will be people who do not remember the dark ages of the early Nineties and swingeing cuts," he said.
"We do need to focus clearly on what could be an austere future, but we are not here to plan for reductions in funding. We are here to fight for education funding and hope that the DCSF is doing the same."
Mr Brookes rejected the claim that primaries are guilty of worse financial planning than secondaries.
But he agreed that smaller primaries can struggle to make the same "economies of scale" available to larger schools.
The difficultly is those savings can make staff in smaller primaries more vulnerable, Mr Brookes added.