The plan to get all primary school years in England back to school before the end of term is set to be dropped by the government.
The "ambition" was for all primary school children to return before the summer holidays, the Department of Education said.
However this is no longer thought to be feasible, and Education Secretary Gavin Williamson is due to deliver a statement to Parliament on the wider reopening of schools.
Anne Longfield, the Children’s Commissioner for England, told BBC Breakfast: “I think it’s a huge disappointment for those children who’d expected to go back into school before the summer now now may not.
“It does mean that the vast majority, probably about eight million children, very likely won’t return to the classroom until September, which means that, again, there will be a huge variation in their learning over that period.”
Ms Longfield said she is “incredibly concerned” about the long-term impact on children’s education and wellbeing.
“We know that there’s a real variation in learning. We’ve got some children, more affluent children, especially those going to private schools, who are literally attending zoom schools from nine till three in the afternoon with lessons as normal," she said.
“And we know that 90% of disadvantaged children aren’t going online for more than two hours, if that."
Geoff Barton, general secretary of the Association of School and College Leaders, said he was not surprised the plan to bring back all primary pupils before the summer holidays was due to be dropped.
He said: “The ‘ambition’ to bring back all primary year groups for a month before the end of the summer term was a case of the government over-promising something that wasn’t deliverable.
“It isn’t possible to do that while maintaining small class sizes and social bubbles, so we aren’t surprised that the policy has been jettisoned.”
Meanwhile, the Minister for care Helen Whately told ITV News the government has been taking "cautious steps" to get children back in school, knowing that "school is a better place for children to be."
"Particularly those from disadvantaged backgrounds, the longer they are out of school, the greater the risk they will fall behind," she added.
"We do want to see more children go back, but we also know it's not easy for schools and we're listening to schools and headteachers to make sure we have the right policy."
Ms Whately said the government is “following the scientific guidance on what is the safe thing to do and the right thing to do” when it comes to getting children back to school.
She told BBC Breakfast that getting children back to school is “not easy”, adding: “Asking children to social distance for instance is not a straightforward thing to do.”
Ms Whately, who has two children in primary school, said the public will hear more later from Mr Williamson “about the plans over the coming weeks”.
At the Downing Street coronavirus briefing on Monday, Health Secretary Matt Hancock conceded that secondary schools in England may not fully reopen until later than September.
Mr Hancock said it was still “our current working plan” that secondary schools in England will not open until September “at the earliest”.
With the number of new deaths falling to the lowest reported since lockdown began, Mr Hancock said plans to further ease restrictions including the reopening of non-essential shops from Monday could go ahead.
“When you look across the board, it is clear that coronavirus is in retreat across the country,” he said, as deaths linked to Covid-19 reached nearly 51,000.
But with the phased reopening of England’s schools having begun last week, he acknowledged older pupils could still face months without attending class.
“That is our current working plan, is that secondary schools won’t open until September at the earliest,” he said.
Children in England began returning in a phased process last week, with Reception, Year 1 and Year 6 pupils heading back first.
Mr Hancock has also unveiled plans for pupils and teachers across England to receive coronavirus testing to monitor the spread of the disease as classes resume.
With approval from parents and guardians, children will be tested to see whether they have Covid-19 or have had an infection in the past under the surveillance programme.
Mr Hancock is aiming to have up to 100 schools tested across England by the end of the summer term, with around 200 staff and children involved at each of those schools.