Angry rhetoric from the Government designed to put headteachers off taking part in the Sats boycott may have actually encouraged more to take part, it has been suggested.

A snapshot poll of school leaders by The TES this week indicated around 44 per cent of schools could be taking part, 49 per cent would administer the tests and 7 per cent are still undecided.

A handful of local authorities say up to half of their primary schools have reported they are taking part, although at the time of going to press, most said full figures were unavailable.

In Kirklees, the eighth largest local authority in England, 83 schools out of 152 have said they will not be sitting the papers. In Dudley, around 50 out of 79 primaries have said they are backing the boycott.

These findings almost match union estimates of around 50 per cent of the 16,000 eligible schools joining in the action, despite a last-ditch attempt by Schools Secretary Ed Balls to scare heads off.

In a letter to school governors last week he warned that it was “unacceptable” for children not to sit the tests, and said he hoped heads would “think twice before disrupting children’s learning, confusing and inconveniencing parents and damaging the profession’s reputation”.

Rod Woodhouse, a headteacher who was undecided until this week, said the stance of the Government had made him make up his mind.

Mr Woodhouse, head of the 103-pupil Essendon Primary School near Hatfield in Hertfordshire, said: “Feeling slightly contrary, the more pressure I feel is being put on me by Government, the more bloody minded I feel about it.”

He said he was disappointed by Mr Balls’ attitude at the recent NAHT conference, where he passed up an opportunity to be “conciliatory”.

“We’ve tried to be professional, so when someone waves a big stick at you, it smacks of desperation,” he said.