Plans to make schools provide full-time alternative education for pupils excluded for more than six days have been labelled unworkable by heads, who said the rule could lead to more permanent exclusions.
The guidance now under consultation would mean that keeping in touch and sending work home to excluded pupils would no longer suffice.
But delegates at the National Association of Head Teachers' annual conference in Bournemouth last weekend said schools lacked the resources to cope with the proposed higher level of provision.
Clarissa Williams, vice president of the association and head of Tolworth girls' school in Surbiton, Surrey, warned that permanent exclusions could soar.
She said: "Heads will not be able to manage and many will take the option of permanent exclusion because responsibility is then passed on to the local authority."
Mrs Williams also said the Government had not been clear on what would be expected under the guidelines, which could come into force in September under the Every Child Matters agenda.
She called for the Government to rethink the guidance and provide greater resources for specialist units and staff.
"Schools could pool resources to fund a special behaviour unit, but we would have to ensure access was available to everyone," she said. "At my school we are converting a mobile classroom, but what we will have to spend is above and beyond our special needs budget."
Since September last year, the association has received around 1,000 phone calls from heads struggling with pupils with extreme behavioural difficulties. The conference passed a series of motions relating to the issue of child behaviour.
Julie Graham, a junior school head from Berkshire, described to the conference what her staff had gone through when three seven-year-olds with severe behavioural and emotional difficulties joined her school last September.
The children trashed classrooms, punched and bit staff, and frightened other children.
Mrs Graham soon realised the school could not meet the children's needs and turned to the local authority. Two of the boys eventually found places in special schools and the third settled down.
She said: "It really is a postcode lottery as to how much support you will receive. I believe strongly in inclusion, but there needs to be a greater understanding of its impact on mainstream schools."