With the general election at the most just eight weeks away, the union focused on class size with a poster showing a teacher facing 37 primary pupils and declaring, “One into 37 doesn’t go”.

Although Labour is committed to classes of under 30 for children under the age of seven, the NUT denied it was urging voters to back the party; the Liberal Democrats are also pledged to cutting class sizes.

“We aren’t saying whom people should vote for,” said general secretary Doug McAvoy. “What we want people to do is to vote for education. What that means about the name against which they put their cross is something each voter must decide.”

The NUT has urged voters to judge election candidates on class size, nursery education, selection, crumbling schools and spending. It wants the parties to be judged on their willingness to extend nursery education to all three- and four-year-olds without vouchers, to oppose selection, tackle crumbling schools and boost spending.

The poster, which is going up in 700 sites across the country, is being backed up by advertising in national and regional newspapers.

Mr McAvoy, while not promoting Labour, hoped the unions could look forward to more genuine consultation with a Labour education secretary. “The first issue to be raised with David Blunkett would be to establish better channels of communication between government and the profession,” he said.

Gillian Shephard, the education and employment secretary, certainly talked to the unions more than her predecessors but the policies she implemented showed no sign of it, said Mr McAvoy.

“What areas has she consulted the profession about where the decision has been anything other than what she has been told it had to be by the right wing of the Tory party?”