Despite teaching moving online in the last year children learned 'little or nothing', according to the research.
A new Oxford University study claims that - despite high-quality remote teaching - primary age students lost a fifth of the progress they would usually make, during the pandemic.
This is equivalent to the time they spent outside the classroom, according to the experts.
The study found children learned 'little or nothing' during school closures - with learning losses even more prominent in families from disadvantaged backgrounds and with low levels of education.
The findings come from data gathered in the Netherlands, where schools closed for eight weeks in the first lockdown, but researchers at Oxford's Leverhulme Centre for Demographic Science believe they're applicable to the UK too.
The study's authors Dr Per Engzell, and DPhil candidates Arun Frey and Mark Verhagen, say the findings are 'dire' and are likely to be reflected internationally - since the Netherlands' students had among the best possible virtual learning opportunities and yet lost on average 20% of expected progress because of school closures.
Dr Engzell said: 'Students made little or no progress while learning from home and losses are particularly concentrated among students from homes with parents with low levels of education. For them, the loss was about 50% worse than for others.'
Arun Frey said: 'These results confirm many of the worst fears that educators and other stakeholders had when going into the first lockdown.
'The Netherlands did so many things right. Teachers and school officials made a tremendous effort. The government provided additional support and financial resources. From broadband access to housing and parental support, conditions in the Dutch system outscore most of its neighbours.
'If anything, consequences are likely to be graver in the UK, where schools also stayed closed for longer.'
Despite advanced digitalisation in Dutch schools, a relatively short first lockdown and world-leading rates of broadband access, student progress was seen to decline considerably.
According to the team, this finding is particularly troubling given that many countries were far less prepared for the challenges of remote learning.
Findings also reveal that, although the vast majority of students' performance deteriorated during remote learning, some schools experienced only a small, or no drop, in learning.
Mark Verhagen said: 'Teachers' actions and school policies seem to play an important role in mitigating the negative consequences of the pandemic, and we need to understand what's driving these differences.'
As students and teachers across the world return to in-person schooling, the researchers say their findings are 'crucial to prepare them for the gaps that will need to be bridged.'