The biggest academy chain in London is preparing to reopen next month, warning of the “social and emotional” impact on pupils of delaying a return until September.

The Harris Federation, which educates 30,000 young people in London, is making plans to reopen to children in reception, years one and year six, as per government guidance.

Education unions have advised teachers and support staff not to “engage with” plans to reopen schools in a phased manner from June 1.

But Harris Federation chief Sir Daniel Moynihan told the Evening Standard: “If children do not have any school until September there will be a cost to them in social and emotional development. For many, they are better off in school.”

The Department for Education wants children back in primary schools starting next month, with class sizes limited to 15 and a staggered timetable to limit risk of transmission

Sir Daniel said that children in reception and year one develop important social skills and the foundations of reading, and missing this time in school will lead to learning loss, especially for more disadvantaged children.

He added that those in year six will benefit emotionally from going back to school, seeing their friends and preparing for the transfer to secondary school.

He said each of the schools in the Harris Federation is undertaking a risk assessment. He said: “OK, the virus was there at the end of March. It will still be there after a 10-week shutdown.

“But our understanding of it is better now. We have had time to think about how to operate and what we can do to reduce risk and there does seem to be a fair amount of evidence that suggests very young children are not great spreaders.”