Secondary school pupils in the UK are falling behind their international counterparts, according to a major survey from the OECD.
Pupils have slipped down a global league table in reading, maths and science, based on two-hour tests taken in 65 school systems around the world.
Finland and South Korea, as last time, achieved the best results
Education Secretary Michael Gove said the survey showed the "urgent need to reform our school system".
Within the UK, Wales performed significantly less well than England, Scotland and Northern Ireland in all three subjects.
The latest findings of the PISA survey - the Programme for International Student Assessment - show the UK tumbling down the rankings, according to the results of tests taken last year by an international sample of 15-year-olds.
The survey by the OECD (Organisation for Economic Co-operation and Development) is designed to compare standards between different education systems around the world.
In 2000, when 32 countries took part in the survey, the UK came 7th in reading skills - but the figures for 2009 show that out of 65 countries and regions, the UK has fallen to 25th place.
Countries such as Poland, Estonia, Canada, Norway and Singapore are above the UK in reading ability, in a table headed by South Korea and Finland.
The study also includes regions within countries - and the Chinese school systems in Shanghai and Hong Kong are among the most successful.
If Shanghai had been a country, it would have been the single most successful in this global survey.
A quarter of pupils in Shanghai were able to tackle complex maths problems, compared with an average of 3% across the OECD survey.
In maths, between 2000 and 2009 the UK has fallen from 8th to 28th and in science from 4th to 16th.
The results for the UK's teenagers have not declined significantly across these years, says the OECD - it is more the case that they have failed to keep up with the improvements of pupils in other countries.
"The UK's performance is about average," says the OECD's Michael Davidson. "The question is whether the UK thinks that 'average' is good enough?"
Mr Gove said the rankings showed that the previous government had not achieved value for money from its investment in education.
"I'm also daunted by the scale of the challenge, because other countries have been improving rapidly and despite massive investment over the last 13 years we haven't been improving at the rate we should have been," said Mr Gove.
The education secretary said that his forthcoming school reforms for England were being influenced by the lessons of successful school systems in other countries.
He said the survey showed the benefits of school autonomy, parental choice, transparency of performance and investing in supporting disadvantaged pupils.
Shadow education secretary Andy Burnham said: "Schools improved under Labour, and more students now leave with good results. People forget how bad things were: in 1997, half of all schools failed to reach the basic benchmark of 30% of students getting 5 GCSEs graded A*-C, including English and maths - that number is now fewer than one in 12."
The 2009 tests also showed that boys continued to achieve lower results than girls, with boys trailing girls in reading in every country - on average falling a whole year behind.
The survey highlighted factors that were seen as characteristics of successful systems.
It found that the "best school systems were the most equitable", in which pupils could succeed academically despite a disadvantaged background.
It also pointed to the importance of good discipline and high status for teachers.
The PISA survey is held every three years, with the results published the year after the tests are taken. In the 2009 survey, about half a million youngsters took the tests, which are designed to provide an objective measure of their ability in these three key areas.
The UK did not take part in all the previous surveys, so there are not continuous figures from 2000 through to the latest published results.