Children in England are three months behind in their studies after lockdown, with boys and poor pupils worst hit, suggests a survey of teachers by an educational research organisation.
The learning gap between rich and poor pupils grew by almost half between March and July, the National Foundation for Educational Research has found.
The authors also warn a quick catch-up is unlikely.
The government says children must not lose out because of coronavirus.
The new term begins in England and Wales this week, after the disruption caused by the Covid-19 pandemic. Schools are already back in Scotland and Northern Ireland.
The National Foundation for Educational Research’s survey questioned a weighted sample of almost 3,000 heads and teachers in about 2,200 primary and secondary schools across England.
The research was carried out just before the end of term in July – and showed how much children had fallen behind by the end of the last school year.
Almost all the teachers questioned (98%) said their pupils were behind the place in the curriculum they would normally expect for the time of year.
Overall, teachers said they had covered just 66% of their usual curriculum by July, putting pupils three months behind in their learning.