A quarter of the 638 schools warned by the government they could face closure have been given the highest evaluation by watchdog Ofsted, it has emerged.
The National Union of Teachers (NUT) has analysed the list of schools and determined only 11 per cent are highlighted by Ofsted as needing intervention.
Schools secretary Ed Balls launched the £400 million National Challenge last week, when schools with fewer than 30 per cent of pupils achieving five good GCSEs including English and maths were warned they would be closed.
The NUT has today written to its members and headteachers of the affected schools to condemn the government's "threats".
It published research on Friday showing a quarter of the schools are considered to be among the best in the country by Ofsted, while half are rated as satisfactory.
"It seems to me that the government has decided to junk its recent attempts to introduce a measure of sophistication into evaluating schools in favour of a crude headline-grabbing measure to try to show that it is tough on standards," commented acting NUT general secretary Christine Blower.
"The support outlined in the National Challenge programme will be obscured by this injustice. Teachers and head teachers will be very wary of wanting to join schools that could be threatened with closure."
A spokesman at the Department for Children, Schools and Families told inthenews.co.uk the NUT was levelling an "old accusation" against the government.
He explained Ofsted was reviewing its classification of schools with an emphasis on results rather than progress or effort.
"Effort and progress are both great but they are no consolation for parents who want a school to get good grades for their children and not just improve."
He added the department would "make no apologies" for warning schools they would be closed if they did not improve.