Charles Clarke has condemned the leading teaching union seeking to block the government's education reforms.

The education secretary described opposition to the expansion of classroom assistants as an "old time religion".

Clarke praised the unions which had supported the plans. "This is the sort of trade unionism I want to do business with," he said.

"Grown-up, forward-looking progressive trade unionism. Trade unionism that is focussed on the big issues and how we solve them.

"Trade unionism that delivers results. I value this new relationship. I value our partnership."

The minister warned that those seeking to obstruct reform would be faced down.

"I have no time at all for the sort of militant opportunism that some people still cling to," he said.

"There are no minority vetoes and old style vested interests will not stop us from delivering change for the better.

"I think that the old time religion has had its day."

The intervention came amid recent signs of rifts within the executive of the National Union of Teachers over its continuing opposition to a larger role for classroom assistants.

But senior union leaders indicated their position would remain unchanged.

"The NUT will not sign the agreement because it allows non-qualified people to take whole classes without the presence of a qualified teacher. That is a matter of principle," said head of education John Bangs.

"The union did not exclude itself from the talks. It never refuses to talk to government. It was excluded by the government.

"But the NUT is an independent, free trade union with the right to agree or disagree with proposals from government in the interests of the teaching profession and the children in our schools."

And the Liberal Democrats also gave their backing to the union.

"Charles Clarke is attempting to bully the NUT into submission and deny the real effect of this year's school budgets crisis. How can schools be expected to implement these reforms when many teaching posts have already been cut to meet the funding shortfall?" asked education spokesman Phil Willis.

"Schools can only implement workload reforms if they are confident that the plans will be fully funded by the department. The DfES must make this commitment clear.

"Given that half of Labour's 25,000 extra teachers are not fully qualified, what price will our children pay for these reforms?"

Under the classroom agreement, from this September teachers with management positions will be allocated separate time outside the classroom to conduct their duties, while they will perform less clerical tasks.

From September next year, teachers will be set a maximum amount of hours during which they are required to cover for absent colleagues at 38 per year.

From September 2005 extra time will be allocated during the school day for the planning of lessons.

"Our goal is to give every pupil the individual attention they need to fulfil their potential," said Clarke.

"We can't do this without radical reform of the school workforce. We have to free teachers to teach and give support staff a bigger role.

"Then every child can get the tailor-made learning they need to make the most of themselves."