Researchers say the results are concerning, as teachers made a 'tremendous effort' during lockdown.
Primary children learned 'little or nothing' at home during the shutdown of schools, according to new Oxford University research.
The study found that, even in a country well-equipped to handle the challenges of a lockdown, remote learning has been 'largely ineffective.'
The Oxford researchers based their investigations on students in the Netherlands, who they say lost about 20% of the progress they would usually make in a year.
The impact of online learning was even more severe for children from disadvantaged backgrounds.
The Leverhulme Centre for Demographic Science at the University of Oxford is behind the study, which is yet to be peer reviewed.
Author Per Engzell said: 'Students made little or no progress while learning from home. Losses are particularly concentrated among students from homes with parents without university education. For them, the loss was about 50% worse than for others.'
Co-Author Arun Frey added: 'These results confirm many of the worst fears that educators, parents and other stakeholders had when going into the first lockdown. The results are particularly concerning as the Netherlands did so many things right: teachers and school officials made a tremendous effort, the government even stepped in and bought laptops for children who needed one.'
The Netherlands offered the Oxford researchers an opportunity to study the effectiveness of online learning, since nationally standardised exams took place just before and after school closures - making it possible to compare progress in the period schools were closed with a normal year.
According to the researchers, the Netherlands provides a "best-case" scenario for studying the effect of school closures on student learning, because of its world-leading rates of broadband access and its short lockdown in the first wave.
Researchers questioned if these results represent a temporary setback, for which schools and teachers can eventually compensate.
Co-author, Mark Verhagen, said: 'Only time will tell if students bounce back, remain stable, or fall further behind. What we know is, if no action is taken, small losses can cumulate into large disadvantages over time. It is on us as a society to step up our efforts if we are not going to fail a whole generation of school children.'
According to the UN, some 95% of the world's students have been affected by school closures, constituting the largest disruption to education in history. Many observers have pointed to the potential harms in terms of learning, and the increased care burdens on parents.
In countries such as England, Germany and France, leaders have opted to close bars and restaurants, while letting schools stay open in a second lockdown. Elsewhere, children are still being kept at home.