Scotland's P1 and P2 pupils are among those hardest hit by school closures which have damaged progress in reading and writing, according to a national equity audit on the effects of lockdown.
Children moving from primary to high school are also very likely to experience a negative impact on their development, research suggests.
The audit, ordered by Scottish ministers to assess the fallout from the first period of remote learning, found that, for some secondary-aged young people, it became normal to sleep all day and not engage until the evening, if at all.
Information was gathered from 54 schools across all 32 local authorities in November last year.
Addressing the issue of attainment, the audit report said: “Most school staff stated that the negative impact of the closure of school buildings was most evident in younger children with the progress of children in P1 and P2 being most notably affected.
“The majority of parents, staff and partner agencies said that children who moved from nursery to primary school or from primary school to secondary school were most likely to be adversely affected by the period of remote learning.”
It added: “A few headteachers indicated that remote learning had been detrimental to language development, particularly for younger children.
“Teachers highlighted the challenge of replicating routines and teaching approaches online, such as the structured repetition required for early level reading and writing.”
Some secondary-aged pupils did not engage until the evening, it was found.
Higher numbers of children and young people from socio-economically disadvantaged backgrounds showed regression in core literacy and numeracy skills when schools re-opened, the audit report said.
Its findings were published on the day Education Secretary John Swinney unveiled a £45 million funding package aimed at boosting Scotland’s remote learning offer.
Councils may use the funding – sufficient to fund 2,000 additional teachers - for purposes including staff recruitment, the purchase of digital devices or to provide extra family support.
Remote learning will also be subject to checks by HM inspectors.
It comes amid growing fears over whether the alternative assessment model announced after school exams were cancelled can be delivered now that classrooms are closed until at least February 1.
Jamie Greene, Scottish Conservative Shadow Education Secretary, said: “While this commitment... to fund more teachers is welcome, it is yet another example of John Swinney finally doing the right thing at the last minute.”
He added: “A phased return for more pupils to return to classroom learning would be welcome and if that is to be the case, the SNP Government need to provide the clearest possible guidance to teachers and parents.”