The government must take urgent action to deal with a "severe shortage" of physics teachers or the subject will die out in schools, a report has said.
Fewer pupils are studying A-level physics
Buckingham University academics studied 432 schools and colleges in England and Wales and found 38% fewer pupils were taking A-level physics than in 1990.
Over the same period, the number of new physics teachers dropped from about a third of the science total to 12.8%.
The government said it was working hard to reverse a "long-term trend".
'More to do'
A Department for Education spokeswoman said: "The number of people starting teacher training in science has actually risen by over 30% from 2,279 in 1998/99 to 2,998 in 2005/06.
"Entries for physics GCSE actually increased in 2005 and at A-level science subjects have not 'slumped' as some predicted.
"But we recognise there is more to do," she said.
Professor Alan Smithers and Dr Pamela Robinson, who conducted the survey, said that although overall A-level entries in all subjects had risen by 14.6% since 1990, the number of physics entries had fallen by 38% (from 45,334 to 28,119).
Nearly 10% of state schools with sixth forms do not offer A-level physics now, and 39.5% had in 2005 five entries or fewer, they found.
The report also found that half of physics teachers had not studied the subject to any level at university, with this being most common among younger teachers.
"Increasing numbers of teachers of physics are qualified in biology, with more aged 21 to 30 holding a degree in biology than in physics," it said.
Independent schools were more likely to have physics teachers who had studied the subject at university level, the researchers said.
The authors concluded: "Physics in schools and colleges is at risk through redefinition and lack of teachers with expertise in the subject.
"There is already a severe shortage. If physics is to survive in schools, both as essential education and a platform for higher level study and research, there is a need for urgent action."
Lord May of Oxford, the president of the Royal Society, said: "The profound problems facing science at A-level extend well beyond physics.
"We have consistently highlighted the general downward trend of students studying the sciences - apart from biology - and maths at A-level. If we fail to address this then we risk losing the ability to train the next generation of scientists, technologists and engineers.
"The government, and particularly the Department for Education and Skills, needs to wake up to the problems facing science education."