Wales' performance has fallen to its lowest level ever in maths, reading and science tests taken by 15-year olds.
In the first international Pisa assessments since Covid, Welsh scores were the lowest of all UK nations and the results gap has widened.
The tests were taken in 81 countries in 2022 and are seen as an important measure of education systems.
Education Minister Jeremy Miles said the pandemic had derailed some improvement in literacy and numeracy.
He said plans had been launched to raise standards.
In maths, Wales' average score was 466 - down from 487 in 2018.
The drop in maths is equivalent to a whole year of study, said the Organisation for Economic Co-operation and Development (OECD), which runs the tests.
All UK nations saw a slump in maths, but it was the steepest in Wales.
Wales is ranked just above the United States, the same as Malta, and just below Norway and Vietnam.
Singapore is at the top (average score 575), followed by Macao and Taiwan.
