More than a third of secondary schools will be forced to make their special educational needs co-ordinators train as teachers to comply with regulations that come into force in September.
The Association of School and College Leaders said the change was a backward step and that schools would struggle to observe the new regulations.
From September, all Sencos must either be qualified teachers or, if they are already in post, be training to qualify by 2011.
The ASCL surveyed 190 secondaries and found that 36 per cent had a Senco who was not a teacher.
John Dunford, general secretary of the ASCL, called for the Government to postpone the requirement until it had completed the whole-scale review of special needs it announced last month. Ofsted also plans to carry out a major review of special needs this year.
Dr Dunford said: "There are many well-qualified, capable Sencos who do not have teaching status. Special needs provision will not be improved by forcing them to qualify as teachers."
This view is shared by Andrew Clary, deputy head of Whitley Abbey School in Coventry, where the Senco is a non-teacher.
"It is a wild and counterproductive assumption that qualified teacher status automatically produces quality staff," Mr Clary said.
But Nasen, the professional body for people working in special needs, has supported the change, as has the National Union of Teachers.
John Bangs, head of education for the union, said heads who opposed the move were "utterly wrong". "Special needs pupils are being short-changed," he said. "Teaching assistants are doing their best, but it is a two-tier system and the pupils who need the most support are not getting it."
Lorraine Petersen, Nasen's chief executive, said it was implausible that the Government's review would overturn the decision, making its postponement pointless and frustrating.
"Who knows how long the Government's review will take? Another five years?" she said. "We fought for this change for so long - we shouldn't have to wait any more."
The ASCL survey found Sencos had a mix of responsibilities, with 73 per cent saying the role included teaching. Administrative tasks were carried out by 81 per cent, 38 per cent managed teaching staff, and 91 per cent were involved in the assessment and identification of special needs.
The figure for non-teaching Sencos is likely to be lower in primaries, where most have one in their senior leadership team.
A study published last year by Leeds University suggested that 9 per cent of Sencos were not qualified teachers, but the national figure is expected to be higher.