Head teachers face legal action from their staff if they fail to implement a deal designed to free up teachers' time, Education Secretary Ruth Kelly has warned.

The biggest head teachers' union last week voted to pull out of the controversial workforce reform agreement, insisting they do not have enough money to make it work.

But Ms Kelly, MP for Bolton West, dismissed heads' complaints over funding as a "smokescreen".

She told the Association of Teachers and Lecturers annual conference in Torquay: "This will be law from September and they have to implement it. We won't tolerate any excuses."

She said teachers would go to industrial tribunals to secure their legal entitlements if heads refused to implement the deal.

The agreement is designed to make sure all classroom teachers in England get 10 per cent of their working week set aside for planning, marking and other tasks.

Mary Bousted, ATL general secretary, welcomed Ms Kelly's comments. She said: "If heads continue to refuse to implement the agreement the course is clear. We will take them to an industrial tribunal and we will win.

"I strongly advise those head teachers who are still saying they can't implement the agreement to stop whingeing and start planning."

But David Hart, general secretary of the National Association of Head Teachers, said: "It's all very well the Secretary of State wagging the finger at the nation's head teachers.

"But to argue that funding is a smokescreen is completely wrong. Funding is a real issue."