Plans for rapid changes to A-levels are "high-risk" and a "huge gamble", elite independent school heads say.
The Headmasters' and Headmistresses' Conference is concerned about A-levels being introduced in 2015 without trialling them first, according to the Times Educational Supplement.
All A-levels will become linear, with exams at the end, under the changes.
The government said details of the new qualifications would be available a year before they are first taught.
But Dr William Richardson, general secretary of the HMC, said: "It is a huge gamble to rush so much change at high speed with no piloting."
He added: "It is high-risk. You can't be sure of the consequences."
His warning comes a few days before candidates in England, Wales and Northern Ireland get their A-level results, which are not expected to rise significantly and may even drop slightly as a clampdown on grade inflation continues.
Dr Richardson, whose organisation represents schools such as Eton and Harrow, predicts the reformed A-levels - championed by Education Secretary Michael Gove - will continue a trend towards lower grades.
"If you enter students for three linear A levels, the assessment regime is less predictable for them in their weakest subject because they have had no milestones along the way to help them calibrate their achievement."
This is in part because the current system, where the first year of A-level, known as the AS, counts as a separate result in its own right is being dismantled in the changes.
But it is the "accelerated timetable" of the reforms that most worries the HMC.
A Department for Education spokesman said: "Academics at our best universities have long been concerned that there are real problems with current A-levels.
"Our reforms will raise standards. Ofqual is aiming to make details of reformed A-levels available in autumn 2014, a year in advance of first teaching so schools will have plenty of time to prepare."
Ofqual, England's exams regulator, has just published its plans for the coming exam reforms, including those to A-levels.
It said it did not plan to recalibrate A-level performance standards in the same way as it will materially change GCSE standards.
But Ofqual said that as new A-levels were introduced it would look at making the number of high grades (As and A*s) as "comparable and consistent as possible" from year to year.
Chief regulator Glenys Stacey said: "This is a very interesting time for qualifications, with real opportunities to deliver materially better qualifications, both general and vocational, that will better prepare people for the next stages in their education or careers.
"We are committed to keeping a tight grip on standards, so people can have confidence that qualifications represent a proper record of knowledge, skills and achievement, and to making sure qualifications deliver what their users - students, employers, teachers, universities - need from them."
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