Achievement of Curriculum for Excellence Levels (ACEL) data for 2020/21 lays bare the disruption caused by the coronavirus crisis.
It shows the number of primary pupils achieving the expected levels in literacy and numeracy dropped by 5.4 and 4.4 percentage points respectively between 2018/19 and 2020/21.
Meanwhile, the attainment gap between youngsters in the most and lead deprived areas grew as the poorest were hit hardest.
The attainment gap in literacy increased from 20.7 percentage points in 2018/19 to 24.7 in 2020/21.
In numeracy, it widened from 16.8 percentage points in 2018/19 to 21.4.
Figures show 75 per cent of primary pupils achieved the expected Curriculum for Excellence level for reading and 83 per cent for listening and talking.
Meanwhile, 70 per cent achieved it for writing and 75 per cent for numeracy.
Girls outperformed boys at all stages. The largest difference was in writing, where girls outperformed boys by 12 percentage points.
Education Secretary Shirley-Anne Somerville said: “The ACEL statistics reflect, in stark terms, the significant impact the pandemic has had on our children and young people.
“Before the pandemic, the year-on-year trend in the ACEL data was positive.
"Unfortunately, the disruption caused by Covid-19 presented serious challenges for learners not just in Scotland but internationally.
“Improving educational outcomes is at the heart of our education recovery work, which is continuing at pace.
"This includes recruiting 3,500 additional teachers and 500 support staff over this Parliamentary term.
"We also continue to press on with our mission to tackle the poverty-related attainment gap, backed by a record £1 billion investment.
"Later today, in a statement to Parliament, I will highlight our new and ongoing work to support numeracy and literacy in our schools.
“Since the start of the pandemic, we have committed half a billion pounds to support education, and other data published today reflects some of the progress that has been made.
"The 2021 Summary Statistics for Schools in Scotland show that there are now over 2,000 more teachers than before the start of the pandemic.
“The additional staff have, so far, brought the ratio of pupils to teachers to 13.2 - its lowest since 2009, directly supporting children by increasing the amount of teacher attention available to each child.”