Too many pupils with special educational needs and disabilities in England lack crucial support, a poll of education staff suggests.
More than 80% of almost 600 staff, who responded to an Association of Teachers and Lecturers poll, said some pupils were being failed.
Ministers should provide "significant and immediate" extra cash, said ATL General Secretary Dr Mary Bousted.
The government said it wanted all children to reach their potential.
Of the 585 staff in English state schools who responded to the poll:
70.7% believed the system was failing to identify all children with special needs quickly enough
58.4% believed pupils officially identified as having special needs did not receive the help they need to achieve their potential
Almost half (48.6%) said they had been unable to access the support and training they needed to meet their pupils' needs.
The survey follows a debate at the union's annual conference in April this year where delegates expressed fears that the identification and support of special needs pupils was being compromised.
Overhaul
A new special educational needs and disabilities code of practice came into force in England's schools two years ago.
Previously, the parents of special needs children had to ask for a local authority assessment, with the most severely affected children receiving "special needs statements".
At the time the government described the change as "a landmark moment", with "a simpler and more joined up system" stretching from birth to the age of 25, allowing education, health and care plans to replace special needs statements.
But many children with special educational needs or disabilities are now no longer eligible for government funding or support, according to 43% of those polled.
The union says new criteria mean that while pupils with complex or severe needs are eligible for high needs funding support, those with less complex needs such as dyslexia or dyspraxia do not automatically receive support.
As many as 200,000 previously identified as having special needs were not transferred into the new system, says the union.