Schools are facing "knock-on pressure" as demand for special-needs support rises, England's Education Secretary, Damian Hinds, is expected to admit to head teachers later.
Speaking at the National Association of Head Teachers annual conference, Mr Hinds will ask for their advice on changes to special-needs funding.
NAHT general secretary Paul Whiteman called the current picture "bleak".
But he added: "The solution is simple - more money from the Treasury."
'Pressures on schools'
Government changes mean "thousands of children with the most complex needs are now receiving more tailored support to help their learning", Mr Hinds is expected to tell the conference.
"That support needs investment and, while we have already hugely increased spending in this area, I recognise that providing for additional complexities can put additional pressures on schools."
Mr Hinds is expected to announce a call for evidence from schools, colleges and local authorities on how special-needs funding in England could become more effective as the needs of children change and the support schools provide becomes more tailored and complex.
"Teachers change lives, we all know this, and nowhere more so than in the incredible work they do to support children with special educational needs and disabilities.
"They have my huge admiration and thanks for that work," he is expected to tell the conference.
He will say he wants "the best understanding" of how the system for funding high-needs children is operating on the ground, "and whether there are improvements we can make so every pound of public money we spend is building opportunities for young people".
The government says more than a quarter of a million pupils with the most complex needs have benefited from personalised education, health and care plans since 2014, with the high-needs budget rising from £5bn in 2013 to more than £6bn now.
While welcoming Mr Hinds's focus on special needs, Mr Whiteman said the 2014 changes had been hard to implement because of the wider squeeze on school budgets.
Last year, NAHT research found heads reporting months of delay in receiving funding, even for children with conditions such as cerebral palsy needing one to one support.
"The picture facing schools supporting children with special education needs is bleak," said Mr Whiteman.
"Not only are school budgets at breaking point, there have been severe cuts to local authority health and social care provision.
"Schools are left struggling to meet the needs of our most vulnerable pupils."
And Adam Boddison, chief executive of the National Association of Special Education Needs, said: "The aspiration of the reforms has been difficult to achieve in practice because of budget constraints in both schools and local authorities.
"School leaders want to meet the needs of learners but too often they have not had the necessary resources available to them".
Mr Boddison said he hoped the government would increase funding sufficiently "to genuinely meet the needs of all learners, rather than just plugging wider educational funding gaps".
The call for evidence will run until 31 July and will considered as part of the chancellor's review of government spending.
Source: BBC News