By Sean Coughlan BBC News Online at the NUT conference

The demand for a 35-hour working week for teachers has been dismissed as "potty" by the education secretary.

Estelle Morris said that professionals did not put artificial limits on working hours - and flatly rejected teachers' unions calls for the 35-hour week in term-time.

And she hinted that bids to the Chancellor for funding a reduction in teachers' workload would need a clear return on the investment.

The education secretary made the comments at the National Union of Teachers' conference in Bournemouth - where her speech to delegates had been interrupted by heckling.

Industrial action

An alliance of teachers' unions is threatening industrial action in a campaign to reduce workload.

Teachers in England want a deal similar to the 35-hour week agreed in Scotland.

Estelle Morris: Talking tough on strikes

But Ms Morris showed little sympathy for a fixed working week, saying that was "not the mark of a profession, teachers are not an hourly-paid workforce".

"You can't run a school with teachers clocking on and clocking off, it has to be more subtle and flexible than that."

Instead she said that workload needed to be reduced by ensuring that teachers did not take on non-teaching tasks, such as photocopying and collecting dinner money.

"We have to find ways of cutting out the extra, non-teaching tasks, and building preparation time into the working day."

It is expected that extra classroom assistants and other support staff would take up the non-teaching tasks, allowing teachers to prepare for lessons.

Re-inventing teaching

And she warned that as she was asking the Chancellor for "lots of money" to fund such reforms, the"investments have to make a difference".

An independent review found that teachers in term time worked an average of 53 hours a week - and Ms Morris said that 20% of this work was in non-teaching tasks.

Suggesting that there could be a future re-definition of a teachers' role, she said that the modern teaching profession needed to concentrate on teaching.