Reforms to England's A-level examination system are "high risk" and could lead to invalid results and significant drops in the numbers of students achieving top grades, principals of elite private schools have warned.
The school leaders are concerned that the revamped A levels, which are due to be introduced from 2015, will not be trialled before being taken by tens of thousands of young people, despite a pledge to do so.
"It is a huge gamble to rush so much change at high speed with no piloting," said William Richardson, general secretary of the Headmasters' and Headmistresses' Conference (HMC), which represents elite institutions including Eton College and Harrow School.
Dr Richardson's warning comes less than a week before this year's A-level results, which are not likely to rise significantly, or at all, as a clampdown on grade inflation continues. Several top universities are preparing to take part in the clearing process for unfilled places in case they struggle to fill places allotted to high-grade students for a second year running.
Dr Richardson is predicting that reformed A levels will accelerate the trend towards lower grades. The changes will mean that modular A levels are replaced with linear qualifications where assessments take place at the end of two-year courses.
"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," Dr Richardson said.
While some universities have become used to selecting from a pool of high- flying candidates with straight As, from 2017 they may have to deal with similar students getting ACE or even BBF, Dr Richardson said.
"Parents will start to say, `The teaching can't have been great because John's elder sister Jane got AAA, and I don't believe that he's any less bright and he's got ACE,'" Dr Richardson said. The HMC is already lobbying universities to ensure that they are prepared to take account of the new conditions and change their typical offers.
Professor Alan Smithers from the University of Buckingham said the lower grades predicted by Dr Richardson are "likely". He warned: "It may take universities a while to adjust their sights."
But Malcolm Trobe, deputy general secretary of the Association of School and College Leaders, said that he expects exams watchdog Ofqual to ensure that students are not disadvantaged by the reform. "If there was a drop in grades then universities would lower their offers to get (A-level) candidates in," he added.
Last year was an example of how that process can go wrong. Some elite Russell Group universities were left with hundreds of unfilled places as the proportion of students achieving the top A-level grades fell for the first time in 21 years.
But it is the "accelerated timetable" of the reforms that most concerns the HMC. "It is high risk," Dr Richardson said. "You can't be sure of the consequences."
In 2009, Ofqual wrote that "new national qualifications should not be introduced without first being piloted in full", and that "normally this will take two years". But the September 2015 introduction of new A levels, with schools being given a first look at the exams a year earlier, makes that impossible.
Dr Richardson's concerns are heightened because new linear GCSEs are to be introduced at the same time. "Going linear at both levels simultaneously is increasing by another factor the uncertainty about whether the assessments are going to be valid and reliable," he said. "There is so little precedent for anybody to go on when you sit them in the first year, no past papers. And the teaching is experimental - the majority of teachers in all schools under 40 (have) never taught linear programmes."
Ofqual expects next week's domestic A-level results - the fourth summer of the current syllabus - to be "in a fairly stable state" compared with GCSEs, where there are big changes in entry patterns.
A Department for Education spokesperson 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 have plenty of time to prepare."
The changes to A levels begin next academic year, when the usual winter A- level exam session will no longer take place.
But many A-level students outside England will escape the upheaval because they take international versions of the qualification.
In February, exam board Edexcel announced that candidates taking its international A level would avoid England's reforms and be able to opt for a modular qualification that still includes winter assessment.