Lockdown school closures could wipe out 10 years of progress in closing the achievement gap between poor and rich pupils, a report suggests.
Modest estimates in the government-commissioned report suggest the shutdowns could cause the gap to widen by around a third of what it is now.
This could mean poorest primary pupils, who are already nine months behind, slipping back a further three months.
The Education Endowment Foundation study said catch-up tuition would help.
The charity’s research also warned of a risk of high levels of absence after schools formally reopen and that this posed a particular risk for disadvantaged pupils.
The rapid evidence assessment drew together evidence on 11 studies from a number of countries on the impact of school closures, focussing on those which looked at learning loss over the summer holiday period.
It found the estimated impact on the gap between the poorest group of pupils, and their wealthier peers ranged widely from 75% to 11%.
The median estimate was 36%, although the researchers said there was high level uncertainty about this average.
The report is published days after a small proportion of the school population returned to lessons.
Although effective remote learning would limit the extent to which the gap widens, the report said there would still need to be sustained support for disadvantaged pupils to catch up.
Over the past decade, the Department for Education has focused attention and resources on closing the disadvantage gap.
It has narrowed from 11.5 months in 2009, at the end of primary school to 9.2 months in 2019.
Sir Peter Lampl, chairman of the EEF, said: “School closures are likely to have a devastating impact on the poorest children and young people. The attainment gap widens when children are not in school.
“There is strong evidence that high quality tuition is a cost-effective way to enable pupils to catch up.”
His organisation has teamed up with a number of other organisations to run a trial in which 1,600 disadvantaged pupils around England are offered one-to-one and small group tuition.
Education Secretary Gavin Williamson said being in school was vital for children’s wellbeing.
He added: “This innovative online tuition pilot is an important part of our plans to put support in place to ensure young people don’t fall behind as a result of coronavirus, particularly those facing disadvantages.”
Russell Hobby, head of teacher training charity Teach First, described the potential loss as “tragic”.
“This is deeply unfair for these children. We must tackle this head on, standing behind the teachers and schools fighting this injustice daily.”
This should start with intensive catch-up provision when possible, he said, adding more resources need to be targeted towards those pupils who have suffered the most.