Reducing class sizes must be a top priority of a new government, according to teachers.

Some schools are still "hideously overcrowded" with more than 30 pupils to a class, according to the National Union of Teachers.

Their poll of 714 teachers found almost six in ten (59 per cent) believe reducing class sizes should be among the highest priorities for the next government.

Speaking at the NUT's annual conference in Liverpool, general secretary Christine Blower said: "There are still schools that are hideously overcrowded, trying to fit 30 children in a class."

In 1997 Labour pledged in their manifesto there would be no Key Stage 1 class (five to seven-year-olds) with more than 30 children in it.

But Ms Blower said while they cannot not say Labour has not fulfilled their pledge, this was never enough.

Class sizes still get bigger as children progress through school, she said.

Ms Blower said the NUT's aim for was classes of 20 by 2020 to give children "the best opportunity."

Experience shows being in a class of 34 or 35 children "isn't as good as being in a class or 27 or 28," Ms Blower said.

NUT deputy general secretary Kevin Courtney said part of the problem is budget cuts.

"We are already picking up stories of cuts happening in school budgets and rises in class size," he said.

At one primary school in Gloucestershire two classes have been merged to create one of 36 children, as pupil numbers are falling and cannot sustain two classes.

The poll also found just a third (36 per cent) of teachers believe they are trusted to do their job by the government.

Ms Blower said: "I think that teachers do not feel in a general sense they are sufficiently valued by government."

Liberal Democrat schools spokesman David Laws said: "It is disgraceful that thousands of young children are in classes so big that they are illegal.

"Huge classes are more difficult for teachers to control and young children can miss out on the individual attention they need.

"The Liberal Democrats will give schools the money to cut class sizes and not allow them to grow even bigger."

Schools Minister Vernon Coaker said: "Over the last 10 years Labour has massively increased the number of adults teaching children, meaning smaller class sizes and more individual attention for pupils.

"There are now 41,000 more teachers and over 120,000 more teaching assistants than 1997 - the biggest school workforce ever.

"We are determined to build on that with a guarantee of one-to-one and small group tuition for children falling behind in the 3Rs.