Average class sizes have risen in nearly nine out of 10 constituencies since 2010, according to research by the country's biggest teaching union.

Out of 533 constituencies in England, 474 have seen an increase in their average class size since the start of the decade, NEU research shows.

Average class size has fallen in just 59 constituencies over the same period.

The findings come as one in three teachers  have named "reducing class size" as the top priority for the next government in an NEU survey.   

Revealed: The 20 areas with biggest class sizes

Background: Half a million primary pupils are taught in 'supersized' classes

Class size: The 20 areas with the smallest class sizes

Teacher recruitment: Class sizes rise as recruitment crisis bites

Research: Class size: Is smaller really better?

Almost a million pupils – 961,127 – are in classes of 31 or more, up from 747,531 in 2010, according to the NEU. This is a 31 per cent increase over the past nine years.

There are now 63,566 pupils in classes of 36 or more, up from 44,093 in 2010 – a 42 per cent rise.

Class sizes on the rise

The figures also show that, since 2010, there has been a 258 per cent rise in the number of secondary pupils in classes of 36 or more – from 6,107 to 21,843.

Mary Bousted, joint general secretary of the NEU, said: “Pupils are experiencing the inevitable result of several government policies which have conspired to put a squeeze on schools.

"The real-terms funding crisis has had catastrophic effects, including a direct impact on class size. 

“In addition, teacher recruitment continues to lag behind the rise in pupil numbers, especially in secondary, with not enough done to attract subject specialists to the profession.

"Government targets are missed year upon year. A continued depression in teacher pay combined with high levels of workload means the government has been vastly more effective at driving people out of teaching than in retaining them.

“Voters will scrutinise manifestoes closely, and it would be foolish of any party to underrate the impact of education pledges, not to mention legacy, on the result. In this election the public knows that to value education, they must vote for education.”

The Department for Education was contacted for comment.