A report by the Office for Standards in Education said that heads believe that demands on them have increased as a result of the workload agreement, which allows classroom assistants to take lessons to give teachers time for marking and lesson preparation.
The inspectors found most teachers and senior managers were "resolutely opposed" to allowing assistants to teach whole classes, but did welcome an increased role for them.
About half of secondary schools and most special schools already give teachers the 10 per cent non-contact time promised by 2005-6.
But many primary and middle school heads, particularly those facing falling rolls, do not believe they have enough money to implement it. "Few of the primary schools provided teachers with the proposed planning, preparation and assessment time. Most of the headteachers viewed their ability to do so with some scepticism and considered they would be unable to do so without additional funding," the report said.
Some heads took on more teaching in an effort to reduce the burden on their staff.
David Bell, chief inspector of schools, said: "Many of the secondary schools inspected are well on the way." But, he added: "There is too much variation in how far schools have come."
The problems of primary heads were aired in a "passionate" meeting last week of the National Association of Head Teachers' council. While it voted by 29 to 13 to continue to be part of the workload agreement, many said they did not have enough money to implement the deal.
The union's national conference in May said that remaining in the agreement should be dependent on "clear, adequate and direct" funding by the end of the year.
David Hart, NAHT general secretary, said he would be writing to ministers about funding and the quality of some of the training for the agreement.
But he said: "We cannot represent our members' interests from a position of splendid isolation outside the negotiations."
Sid Willcocks, council member for Dorset and Wiltshire, said: "We lost the battle but the war is still going on."
Ofsted's report was based on visits to 25 primary, 20 middle, 10 special and 45 secondary schools during autumn and spring terms 2003-4.
Despite the reluctance of some teachers to delegate tasks such as collecting money and putting up displays, schools have made "satisfactory progress" in transferring non-teaching activities to support staff, it said.
Chris Keates, general secretary of the National Association of Schoolmasters Union of Women Teachers, said the report was out of date and that considerable progress has since been made.
However, a new row is brewing over pay for higher-level teaching assistants, who will be able to supervise classes. The support staff union, Unison, threatened to strike if employers continue to advise that these assistants be paid extra only when they actually carry out specific duties.
News 6, Leader 24