Children around the world have lost more than a third of the standard global 190-day school year because of the coronavirus pandemic, Save the Children said on Tuesday.
The London-based charity urged governments and donors to take swift action to prevent "an irreversible impact" on the lives of millions of children who now may never return to school.
It estimated children have missed out on an average of 74 days of education each due to school closures and a lack of access to remote learning caused by the global health crisis.
The charity calculated 112 billion school days have been lost in total worldwide, with the poorest children disproportionately affected.
Children in Latin America, the Caribbean and South Asia missed out on almost triple the education of those in Western Europe, it added.
"Almost a year after the global pandemic was officially declared, hundreds of millions of children remain out of school," said chief executive Inger Ashing.
"2021 must be the year to ensure that children do not pay the price for this pandemic.
"We will lose the war against the pandemic if we do not ensure children get back to school safely, have access to health services, have enough to eat and are protected."
The charity said supporting children's safe return to school should be made a priority at this year's meeting of G7 wealthy nations, hosted by Britain in June.
- 'I cry' -
Save the Children found at the peak of the pandemic last year, 91 percent of the world's learners were locked out of schools.
Its analysis concluded that the lockdowns have widened the wealth and opportunity gap both between and within countries.
"The divide grew between wealthier and poorer families; urban and rural households; refugees or displaced children and host populations; children with disabilities and children without disabilities," Save the Children added.
It interviewed Santiago, a 13-year-old Venezuelan who attends a school for children with a profound hearing loss supported by the charity and reported being "sad, worried, and scared" by the situation.
"I like school. People understand me there. When I can't go to school, I cry and just want to sleep," he was quoted as saying.
Save the Children also discovered "huge discrepancies" in remote access to learning in wealthier nations during the pandemic, in particular in the United States.
Students there are more disconnected from the internet than those in other high-income countries, likely hindering their online learning, it said.
Meanwhile in Norway, 30 percent of youths aged nine to 18 did not have access to a computer at home, and in the Netherlands this was one of five children.