The leader of Britain's biggest teacher's union has criticised Education Secretary Ruth Kelly's "obsession" with giving parents more power over their children's education.

Steve Sinnott, general secretary of the National Union of Teachers, said Ms Kelly must prove she was as committed as her predecessor Charles Clarke to tackling the problems faced by poor families.

But she will not achieve this by offering parents more "choice" over schools because many working class families will miss out, he said. Mr Sinnott's remarks, at the NUT annual conference in Gateshead, follow comments from the union's president, Hilary Bills, on Friday.

She said Ms Kelly was the worst Education Secretary since Labour came to power in 1997 and lacked the "interpersonal skills" to do the job.

Mr Sinnott focused on Ms Kelly's policies, particularly Labour's promise to give parents a greater say in their children's education.

"Schools and teachers try to compensate for the effects of deprivation," he said.

"They want to provide disadvantaged young people with the support that is absent at home - access to books, access to computers, personal tuition, entitlements to school trips and study visits, and nourishing school meals.

"Charles Clarke and his ministers had begun to express this clearly, but now the Government seems obsessed with the concepts of parent power and choice," he said. "The problem with that, is that divisions in social class make the idea of school choice and parent power an illusion.

"Choice rests with those who have the power to make choices. I want the Government to put the equality agenda back as a priority.

"I urge Ruth Kelly, like her predecessor, to declare her commitment and say she will be steadfast in tackling the effects of social class on children's education."

Under Labour's plans, schools will have to involve parents more in discussions about how to teach children.

Ms Kelly also said she wanted children to be taught in small groups - the sort of opportunities normally available only to parents who pay private school fees.

She has also promised that good, popular schools will be allowed to expand to take on more children.

But Mr Sinnott said: "Policies of parental choice and of market forces help create and reinforce disadvantage."