More than a third of primary schools in England are failing to perform to a controversial set of standards, according to figures uncovered by The TES.
Around 6,000 primaries - of an estimated 17,000 in total - are struggling with low results or are failing to make sufficient progress under criteria for the World Class Primary Programme.
Heads' leaders have damned the extent of the project and have warned that it could lead to widespread closures and federations, saying it left schools "disempowered, bullied, harassed".
Angry school leaders also attacked the programme at last weekend's conference of heads' union the NAHT.
The initiative is based on the hugely contentious National Challenge scheme for secondaries and is designed to aid school improvement.
Its main focus is on primary schools where fewer than 55 per cent of children achieve the expected level 4 in both English and maths at the end of key stage 2. But it also covers schools that are deemed inadequate by teaching watchdog Ofsted, have inconsistent results, or are coasting.
The Government has published statistics showing that 1,472 primaries are under the 55 per cent floor target. It has also been made public that around 300 primaries are either in special measures or have a notice to improve from Ofsted.
But previously unpublished figures - which feature in National Strategies presentations to local authorities - show that in addition to these schools, approximately 3,400 are deemed to be coasting and 1,000 to have inconsistent results.
Just 7,700 primary schools are "good" or "outstanding", according to the National Strategies figures. Its estimates do not include the performance of a further 3,200 schools, meaning that the total could be even higher.
The programme was launched at the end of last year with pound;50 million to be invested in school improvement partners and specialist teachers.
Mick Brookes, general secretary of the NAHT, criticised the high numbers of schools identified as needing to improve, describing the extent of the initiative as "complete nonsense".
"It assumes that one-third of schools have headteachers and staff going into schools determined not to improve children's education," he said. "That is just complete science fiction."
He added: "It needn't have been like this if the intention was to identify schools that require positive help. This is clearly the wrong way to encourage schools and the children in them. It just makes them feel disempowered, bullied, harassed."
Primaries that are performing above the standards set out in the World Class Primary Programme will be expected to lend support to those that are not.
Athersley South Primary in Barnsley is being targeted for inconsistency even though it was only 1 per cent under the floor target last year and has a "good" rating from Ofsted.
Steve Iredale, the school's head, said: "This has crept in through the back door and will come as a nasty shock to people. When they have got the 55 per cent sorted then it will move up so the schools in the 55 per cent to 70 per cent group will be next."
All local authorities had to submit plans by the end of March on how they will improve primary schools involved in the programme.
The 12 worst-performing authorities had to finalise their plans by the end of January. As revealed in The TES, these include Derby City Council, where more than eight out of ten of its primaries fell into one of the categories for action.
The Government has said that it is up to local authorities to decide how many schools are in each category and it does not yet have complete figures.
When the National Challenge scheme for secondaries was launched, the schools involved were easily identified after the Government said it was for those where fewer than 30 per cent of pupils achieved five good GCSEs including English and maths.
This led to heads across the country complaining about being "named and shamed".