Britain is facing a shortfall of 18,000 primary school teachers needed to keep class sizes under 30, a report has found.
The think-tank Policy Exchange said that official statistics predicted the number of primary school-aged children would have grown by 500,000 by 2015.
It calculates an extra 18,000 teachers will be needed by then if class sizes are to be kept at their current average of 26.2.
More than 14 per cent of primary pupils are already being taught in classes of more than 31, the report said.
Sam Freedman, head of the think-tank's education unit, said: "It is unlikely that the government will recruit the 18,000 teachers needed by 2015.
"Local authorities will probably use extra pupil funding to increase the number of teaching assistants further.
"Numbers in primary schools have already risen from 41,900 to 105,800 in the last 10 years.
"Given that assistants are untrained and often have a relatively low level of qualifications - only 25 per cent of existing teaching assistants have A-levels - this should be of serious concern to parents whose children will be entering primary school over the next few years."
The report, by professor John Howson, also showed that applications for secondary teachers are falling after a period of growth.
With shortages in key subjects such as maths, science and modern languages, it concludes that recruitment targets for those courses will not be met this year.
Freedman said: "There are a number of possible reasons for this decline including the impact of higher student debt; concerns about getting a job after training and the strength of the graduate labour market.
"But most importantly, the supply going forward depends upon the view of teaching as a career amongst potential entrants to the profession."
Dods Parliamentary Communications Ltd