Primary school pupils should not be told that they are taking exams to prevent feelings of anxiety, Ofsted's chief inspector has said.
Amanda Spielman said headteachers who speak to children about their Sats exams and ask how they are feeling, could make them more worried.
Her comments came as tens of thousands of pupils begin their Sats and GCSEs exams this week.
Ms Spielman does not believe formal tests should be abolished in primary schools, but she said teachers could do more to ensure children as young as seven are not negatively affected by them.
"Good primary schools manage to run key stage tests often with children not even knowing that they're being tested,” the head of schools watchdog said.
She added: "I was in a primary school not long ago where I saw something that did concern me, where the head was going around clapping the year sixes on the shoulder saying 'so are you feeling ok about the test, is everything going well for you?' I thought actually maybe that is well-meaning, but maybe that's actually subliminally encouraging children to feel anxious. So there is something really important about how we do these things."
The comments come amid rising concern that high-stakes tests in schools are having a negative effect on the mental health of young people - especially primary school pupils.
Labour pledged to scrap the Sats exams last month, as well as plans for a new baseline assessment for four-year-olds in English primary schools.
Jeremy Corbyn said the “regime of extreme pressure testing” would be abolished amid reports of children crying, vomiting and having nightmares.
Already thousands of schools have signed up to pilot the government’s controversial baseline assessment for children in the first few weeks in Reception despite opposition.
Asked whether heads should be wary of encouraging children during tests, Ms Spielman said: “Everyone should think about their behaviour. Because the mere act of talking about something, somebody other than a classroom teacher talking about it, might just help to ratchet things up."
She added: “Clearly, testing happens in every system throughout the world. It only becomes a big deal for young children if people make it so for them."
Ms Spielman was talking ahead of the launch of the watchdog’s new inspection criteria.
Under changes to the framework, which will come into effect in September, pupils’ exam results and test data will no longer be a key focus in inspections. Instead, the wider curriculum will be looked at.
Ofsted hopes shifting the emphasis away from performance data will actively discourage schools from “teaching to the test” and keeping troublesome pupils off the books through “off-rolling”.
The framework will see a crackdown on bullying and low-level classroom disruption in classrooms, with schools being judged on the learning environment they provide for students.
Schools are currently judged on ‘personal development, behaviour and welfare’ of pupils, but Ofsted will now bring in two separate judgements on personal development and behaviour and attitudes.
The changes follow a public consultation which prompted more than 15,000 responses.
Ms Spielman added: “The new framework puts the real substance of education at the heart of inspection and supports leaders and teachers who act with integrity. We hope early years, schools and college leaders will no longer feel the need to generate and analyse masses of internal data for inspection.
“Instead, we want them to spend their time teaching and making a real difference to children’s lives, which is why they entered the profession in the first place.”
Education Secretary Damian Hinds said: "Ofsted’s role in the education system is of vital importance - and the introduction of this new framework is a major step in supporting our shared drive to raise standards across the school system so that every child have access to a world class education."
He added: “I want to work together with the sector and Ofsted as this new framework is implemented, especially over the first transitional year, to make sure it is a positive change in our schools system.”