Primary pupils in England fell months behind in maths and reading losing progress made in the autumn.
Pupils fell behind again in the second lockdown, losing progress in maths and reading from the autumn term.
New research for the government also reveals large regional differences across England in learning loss.
It follows the resignation of Sir Kevan Collins, education recovery commissioner for England, whose plans for helping pupils catch up were rejected by ministers as too expensive.
The government has invested £3.1bn and more money may follow in the autumn.
On 4 January, schools in England opened briefly and then closed for a second national lockdown, leaving most pupils learning remotely at home.
This research for the government by the Education Policy Institute shows that by March 2021, primary pupils were almost as far behind as after the first lockdown and summer holidays last year.
In maths, that meant primary pupils were on average 3.5 months behind where children have been in previous non-pandemic years.
For reading, the learning loss by the end of the spring term was an average of 2.2 months for primary pupils.
The analysis shows schools helped pupils make progress during the autumn, but much of that was wiped out when they closed to all but key worker and vulnerable children.
And some parts of the North East and East Midlands were among the areas worst affected.
At the beginning of the autumn term, researchers were able to capture how far behind primary pupils were in maths and reading compared with previous years.
This reveals in the South West, pupils in primary school were 1.5 months behind in reading, and two months in maths.
But in Yorkshire and Humber, the difference was 2.6 months in reading, and 5.8 months in maths.
As the national picture shows that any progress made later in the autumn was lost in early 2021, the regional differences may remain.
The research also confirmed that the poorest pupils had lost more learning than average.
Jon Andrews, report co-author and head of analysis at the Education Policy Institute (EPI) said:
“Our data analysis points to a clear penalty faced by disadvantaged pupils during the pandemic – these pupils have seen greater learning losses than their more affluent peers, which risks widening the overall gap in educational attainment.”
The research has been published just days after the resignation of Sir Kevan who described government plans to help pupils catch up as “half-hearted”.
He said the government response was “too narrow” and was being delivered “too slowly”. He was reported as having put forward plans costing £15bn.
Head teachers said they were “hugely disappointed” by the government ‘s latest £1.4bn Covid recovery package, calling it a “damp squib”.
It followed an existing commitment to spend £1.7bn in England’s schools.
The prime minister said there would “hopefully” be more money to come in the autumn, as part of the government’s wider spending plans.
But the influential economic think tank, the Institute for Fiscal Studies (IFS) is warning that delaying decisions until the autumn risks being too late to help the pupils most in need of support.
“If these losses are not properly addressed, then the long-run costs could easily run into the hundreds of billions as a result of lost skills and productivity,” the IFS says.
The IFS researchers point out that the recovery package is smaller than in other countries.
The IFS says £15bn sounds like a lot of money to spend on education recovery, but spread over three years, it would have averaged about £5bn per year, or just over 10% of annual spending on schools pre-Covid.
“If it genuinely helped to mitigate the future lost earnings of today’s schoolchildren (which could easily run into the hundreds of billions over the coming decades), it could also prove to be a terrific investment,” it said.
Geoff Barton, general secretary of the Association of School and College Leaders, said: “This research provides further stark evidence of the very severe extent of learning loss among pupils as a result of the disruption caused by the coronavirus pandemic, and the particularly severe impact on disadvantaged children.
“It is abundantly clear that a recovery programme is urgently required at a scale and scope to address this massive issue.”
A spokesman for the Department for Education said: “We have committed to an ambitious, and long-term education recovery plan, including an investment to date of over £3bn and a significant expansion of our tutoring programme, to support children and young people to make up for learning lost during the pandemic.”