It’s a chill, sunny winter’s day with seagulls soaring on a stiff breeze, and small children wrapped up against the cold are serving from a kitchen in the outside play area at Friars primary school and nursery at Shoeburyness on the Essex coast. On the menu are soup, jacket potatoes, jelly and juice – all made of mud.

As a teacher observes and questions, these happy four- and five-year-olds are learning through play the foundations of literacy, numeracy and writing: phonics as they sound out the letters on their menu, fine motor skills as they shape mud and numbers as they count the things needed for their “cooking”.

It’s how Elaine Bennett, head of the early years foundation stage and year one here, believes children in their reception year should learn and be assessed. She is furious, along with many other early years teachers, with government plans for compulsory baseline tests of literacy, numeracy and behaviour in the first six weeks of primary school.

The new education secretary, Damian Hinds, will soon find that many teachers and child development experts are fiercely opposed to the government’s plan. They fear it is part of a trend in which children could lose time at play because they will be expected to do more formal – measurable – learning.

A letter from child development experts, including Robert Winston, published today in the Guardian, calls on Ofsted, the schools’ watchdog, to withdraw its report, Bold Beginnings, published in November, which said reception pupils needed to be pushed harder in reading and maths.

And providers who might have been expected to create the new test are declining the opportunity, amid fears it could even be “immoral”.

Bennett cites educational pioneers such as Montessori and Froebel to back up her insistence that you use observation and a wide curriculum to build a picture of the whole child to guide individual learning. Tests, she says, would involve time out from learning, disrupt the settling in period and risk providing an inaccurate picture.

The Department for Education introduced testing for infants in state-run English schools in 2015 in a voluntary trial involving three assessment suppliers offering a mix of approaches. The most popular by far was based solely on observation of children. But the government found it impossible to compare the approaches and abandoned all three. Instead a new £10m trial, based on a one-off test, not observation, is to begin in September, with the aim of introducing it to all infant schools in 2020.

Alice Bradbury and Guy Roberts-Holmes, of University College London Institute of Education, who undertook research with teachers after the original trials, say many factors could have skewed the results, such as whether the child was summer born, spoke English as a first language, or had settled happily into school.

Bradbury says it is “deeply demeaning and deprofessionalising for teachers”. “You don’t need qualified teacher status to administer a computer test – a robot could deliver it. No one who knows four-year-olds would have set it up like this. You need to do assessments over the long term.”

Children at Friars school learning phonics as they sound out the letters on their ‘menu’, motor skills as they shape mud and numbers as they count the things needed for their ‘cooking’. Photograph: Susannah Ireland/The Guardian

Roberts-Holmes says the test risks making social inequality worse, as parents with high expectations will prepare their children, which could mean these infants have a higher score and that higher expectations will follow them throughout their school careers. The opposite could be true for children from disadvantaged homes.

Another problem many early years teachers have with the test is that they won’t be given the results, so won’t be able to use it to help children. The DfE tender document, issued just before Christmas, makes it clear that the outcomes will not be available in detail.

This has led to Early Excellence, the supplier of the most popular observation-based tests used in 2015, to pull out of tendering. Jan Dubiel, its national and international director, says: “We are not opposed to the idea of an entry assessment. But all assessment should support learning. It’s absurd and ridiculous to test purely for accountability.”

He argues that research is clear that testing is unreliable until the age of seven, when the brain enters a new phase of development.

“By the time children are eight or nine they know that in a test the point of the game is to get the right answers. Very young children respond very differently and give you the right – or the wrong – answer in different situations.

“Unfortunately, the government has not engaged with us at any point around baseline even though we have asked to meet them. It’s quite clear that there is ideological opposition to observational assessment.”

The DfE view is that the early years foundation stage framework, which does include observation, helps inform teaching and learning and the baseline tests will inform the government’s big picture on progress in schools over time. In the tender, the government is at pains to point out that the results will not be used to judge teachers or schools.

But many remain sceptical, because the government clearly has concerns about the reception year in some schools. The Ofsted report highlighted that a third of five-year-olds, and half of disadvantaged ones, were not reaching expected standards of development in their reception year. The inspectors recommended more focus on reading, including phonics.

Now an open letter from Keeping Early Years Unique, a grouping of teachers, parents and other education experts, complains the Ofsted report is flawed and biased because the schools used as good examples by the inspectorate were chosen for their more formal approach. Ofsted got the results it wanted, it says.

Early years teachers fear the government would like to shrink the early years curriculum, harming the development of young children. Children as young as three are known to be grouped by ability in some schools.

But a good start at school matters. New research from Durham University’s centre for evaluation and monitoring (CEM), published just before Christmas, showed children who do well in reception perform better all the way to GCSE. Katharine Bailey, director of applied research at CEM, says: “It’s very beneficial. We don’t know what makes it happen – is it the teacher, the leaders, the culture of the school?”

CEM provided the second most popular test in the 2015-16 baseline trials but is unlikely to bid for the new tender. Bailey says: “We think there is an opportunity to give valuable information to teachers with these assessments and we think that now isn’t going to happen. If teachers aren’t given helpful and beneficial results that can help children, it’s verging on the immoral.”