A study found 55% of school staff were either "barely confident" or "not confident at all" of introducing National Qualifications, which replace Standard Grades in 2013/14.
The survey, by the Educational Institute of Scotland (EIS) teaching union, also confirmed previous findings that 90% of teachers felt their workload had increased as they introduce the exams in secondary schools.
There was concern over official support materials, with 55% of teachers rating documents from the Scottish Qualifications Authority (SQA) as "not very helpful" and more than half saying support from national quango Education Scotland was "unsatisfactory".
The Scottish Conservatives also claimed three-quarters of teachers said pupils would be studying fewer than eight subjects in S4, which it argued undermined the traditional breadth of the curriculum.
The findings come as secondary schools in Scotland prepare for the new National Qualifications, designed to fit in with the Curriculum for Excellence (CfE). Supporters of CfE believe it will drive up quality by making learning more interesting and relevant.
However, the EIS said its online survey highlighted the need for urgent action by the Scottish Government, councils, Education Scotland and the SQA to ensure schools could deliver.
Larry Flanagan, EIS general secretary, said: "The fact half of classroom teachers are still expressing lack of confidence shows there remains a considerable amount of work to be done.
"We cannot afford for even a small percentage of teachers to be unprepared as that would mean their pupils would be disadvantaged."
Mr Flanagan said it was a matter of "significant concern" that many schools had been unable to access extra financial support from the Scottish Government, adding: "Not all the money for CfE support has reached the classroom, which raises questions regarding where precisely it has gone."
However, Alasdair Allan, Minister for Learning, painted a more positive picture of the roll-out of CfE, saying: "I visit schools across Scotland most weeks and I see that Scottish education is good, and it is getting better.
"I see inspiring teachers and dedicated leaders rising to the challenge of delivering a modern education for their pupils and working well with parents in doing so."
Dr Gill Stewart, SQA director of qualifications, added: "We understand teachers need a full picture of how the new National Qualifications will work and SQA is committed to delivering that as quickly and comprehensively as possible.
"The release of the specimen question papers next week and the coursework materials that will follow are a big step towards that full picture."