The decline in numbers of students taking A-level physics, chemistry and maths must not be allowed to continue, the Royal Society says.
Science has declined in popularity
In 1991 43,416 students sat A-level physics compared with 28,698 in 2004, a decrease of 34%, with the other subjects also losing popularity.
Royal Society president Lord May of Oxford called for "proper targets".
But the government said science subject applications were up by more than 10% this year and were a priority.
In chemistry, 44,440 students took an A-level in 1991, but by 2004 this had fallen to 37,254 - a drop of 16%.
The number of maths candidates decreased by 22%, from 74,972 to 58,508.
However, there were 10% more A-level entries across all subjects in 2004 than in 1991.
Lord May said: "If the number of students studying chemistry, physics and maths at A-level continues to decline, we risk losing not only the next generation of highly skilled scientists, technologists and engineers, but also the teachers to train the generation that come after them.
"And without the demand from students we may face a situation where some schools actually stop offering some science subjects at A-level."
The government has set out aspirations to raise numbers of students taking science and engineering after the age of 16.
Lord May said: "The government must commit to some proper targets and a route map detailing exactly how and when it intends to achieve its targets of increasing the numbers of students studying the sciences at A-level and beyond."
Earlier this week the CBI said the UK could lose trade unless science and language learning improve.
A Department for Education and Skills spokesman said: "The sciences continue to be among the more popular courses at A-level.
"The most recent set of figures on applications to science subjects was up by more than 10% compared with last year. We want this to continue."
A programme of science study at GCSE level would help ensure a "clear focus", he added.
The British Chambers of Commerce has demanded higher A-level standards overall.
Its survey of small firms suggests that 50.4% of small firms have trouble recruiting adequately skilled staff.
BCC president Bill Midgley said A-levels had been "dumbed down", with a 97% pass rate expected when results are announced on Thursday.
The government, however, insists that teaching and general standards have improved.