Major reforms to GCSE and A-level exams could cause "incalculable" damage to children's education, headteachers have warned.
The move to assess most subjects through written papers only is a "retrograde step" with exams at risk of becoming "a feat of memory recall," the National Association of Head Teachers (NAHT) annual conference has heard.
Under sweeping government education reforms, new, tougher GCSEs and A-levels are due to be introduced in England from 2015 onwards.
The changes will see ''modular'' qualifications scrapped in favour of two-year courses with exams at the end, less coursework, and a brand new grading system at GCSE.
But school leaders criticised the shake-up, warning that only assessing children through final written papers will give an "unreliable and unrepresentative" picture of a teenager's true abilities.
Delegates at the NAHT's annual conference in Birmingham unanimously passed a resolution urging the Government and all political parties to ensure that new GCSE, A-level and vocational qualifications contain "an appropriate balance between knowledge, understanding and skill acquisition".
Proposing the motion, Alan Mottershead, head of Trinity School in Carlisle expressed concerns over exam reforms, especially changes to assessment.
"The damage that this may do to a broad and balanced curriculum across the secondary phase, in particular in GCSE and A-level years is incalculable," he said.
He warned that final written exams are taking the place of coursework, projects, experiments and practical work in subjects such as PE, art, drama and technology.
"As exams become timed written papers at the end of course only and their content a feat of memory recall we feel also that exams will be even more of an unreliable and unrepresentative picture of a student's work or real ability," he said.
"It will also deny the richness, exploration, curiosity, wonder, questioning and many other things which characterise a real education.
"They will be replaced by 'learn this and make your answer book like that'.
Mr Mottershead warned that many students, such as those who are dyslexic or suffer from exam stress, will not be helped by an exam only system.
"We want in a civilised society an opportunity for each student to do as well as they can and the changes to the exams system need to be brought into to allow that," he insisted.
Rob Campbell, head of Impington Village College, Cambridge said: "We should be preparing young people for a world that is dynamic, demands greater flexibility, and is about enterprise and creativity; I believe our curriculum and assessment should mirror these conditions.
"Therefore the planned changes that will see for most subjects the factual recall of limited content via just one mode of presentation, namely a two or three hour exam and no coursework, is a retrograde step and one I deplore."
Speaking afterwards, Mr Campbell said parents are unlikely to be happy with the change.
"I cannot believe parents will want their children assessed exclusively by terminal exam, a once and only sitting of an exam at the end of a course," he said.
Delegates at the conference also backed a new education manifesto drawn up by the union ahead of next year's General Election.
The document included a proposal for poor pupils to be given priority for school places - including at private schools - and the union called for research to be done on the impact of this plan.
It also called for a study into the impact on children's learning of the traditional long summer holiday.
A Department for Education spokesman said: "Our changes to reform GCSEs and A-levels so they are rigorous and demanding will mean pupils develop an in-depth and lasting understanding of a subject and bring to an end the treadmill of constant revision and exams.
"For years employers have complained about the lack of knowledge of school leavers, academics at top universities have insisted A-levels are made more stretching so they prepare young people for degree courses, and we have stagnated in international tables while other countries have surged ahead.
"Our changes will also address the pernicious damage caused by grade inflation and dumbing down, which have undermined students' achievements. And they will give pupils, parents, teachers, universities and employers greater confidence in the integrity and reliability of our qualifications system."