The number of boys doing physics and maths A-levels has dropped dramatically over the past 20 years, research by The TES shows.
Physics entries by boys have fallen by nearly half since 1980, while maths entries have dropped by about 40 per cent.
And girls have not stepped in to take the boys' place. Despite efforts to tempt more girls into physics, their entries also fell by 32 per cent over the period, although girls' maths entries climbed slightly.
Some commentators now fear that boys are turning away from physics because its image has been "demasculinised".
Professor Mary Smyth, dean of applied sciences at Lancaster University, said: "In the past, boys were expected to do science. It was quite unusual for bright boys in grammar schools not to do science."
Now, she said, girls and boys felt felt free to take a wider range of subjects at A-level. The sciences were losing their masculine image and this may have made them less attractive to boys. "In demasculinising them, have we actually made the total number of students drop?" she asked.
At the Institute of Physics, teacher support manager Chris Shepherd said the focus had been on attracting girls into physics. "Given that girls are under-represented, clearly we want to make it as easy as possible for them to take up the course," he said.
He said the institute had not focused on the falling numbers of boys taking up physics: "This is an interesting angle and not one that has been highlighted."
Professor Alan Smithers of the centre for education and employment research at Liverpool University said the fall in the number doing physics A-level was probably due to the move to teaching combined science at GCSE.
Noting that entry figures had stabilised in the late 1990s, after the combined GCSE was introduced, he said: "The most likely explanation is that double award science is a less good platform for separate sciences at A-level."
He said that independent and grammar schools were providing a disproportionate number of physics A-level candidates because they still tended to teach the subject separately at GCSE. Entries from girls had been less affected because a higher proportion of female physicists came from private schools.
Pauline Davies, headmistress of Wycombe Abbey school and president of the Girls' Schools Association, said: "For many years science has been very strong in GSA schools, so as a sector we have produced lots of pure and applied scientists."
Mr Shepherd said a good supply of qualified physicists was crucial to UK science: "The science base relies on well-qualified scientists and engineers, all of which stems from A-level physics. A decline in the number of people taking the A-level has a significant effect."