For a decade we’ve been told that our kids, just as they seem to be getting taller with each generation, are also getting brighter. Every year new waves of children get better GCSE, A-level and degree results than their predecessors. Meanwhile, in primary schools, the standards in national maths and English tests at 11 head in one direction — relentlessly upwards.

Last week came the bombshell that blew a gaping hole in this one-way escalator of achievement.

Far from getting cleverer, our 11-year-olds are, in fact, less “intelligent” than their counterparts of 30 years ago. Or so say a team who are among Britain’s most respected education researchers.

After studying 25,000 children across both state and private schools Philip Adey, a professor of education at King’s College London confidently declares: “The intelligence of 11-year-olds has fallen by three years’ worth in the past two decades.”

It’s an extraordinary claim. But it’s one that should startle parents and teachers out of complacency. Shocked by the findings, experts are questioning our entire exam system and calling for radical changes in the way our children are taught in primary schools.

In their painstaking research project Adey and his colleague, psychology professor Michael Shayer, compared the results of today’s children with those of children who took exactly the same test in the mid-1990s and also 30 years ago. While most exams have changed (been made easier, if you listen to the critics) this one is the same as it was in 1976 when pupils first chewed their pencils over the problems.

In the easiest question, children are asked to watch as water is poured up to the brim of a tall, thin container. From there the water is tipped into a small fat glass. The tall vessel is refilled. Do both beakers now hold the same amount of water? “It’s frightening how many children now get this simple question wrong,” says scientist Denise Ginsburg, Shayer’s wife and another of the research team.

Another question involves two blocks of a similar size — one of brass, the other of plasticine. Which would displace the most water when dropped into a beaker? children are asked. Two years ago fewer than a fifth came up with the right answer.

In 1976 a third of boys and a quarter of girls scored highly in the tests overall; by 2004, the figures had plummeted to just 6% of boys and 5% of girls. These children were on average two to three years behind those who were tested in the mid-1990s.

“It is shocking,” says Adey. “The general cognitive foundation of 11 and 12-year-olds has taken a big dip. There has been a continuous decline in the last 30 years and it is carrying on now.”

But what exactly is being lost? Is it really general intelligence or simply a specific understanding of scientific concepts such as volume and density? Both, say the researchers. The tests reveal both general intelligence — “higher level brain functions” — and a knowledge that is “the bedrock of science and maths” says Ginsburg. In fact it’s nothing less than the ability of children to handle new, difficult ideas. Doing well at these tests has been linked with getting higher grades generally at GCSE.

So why are children now doing so badly? Possible explanations are numerous. Youngsters don’t get outside for hands-on play in mud, sand and water — and sandpits and water tables have been squeezed out in many primary schools by a relentless drilling of the three Rs and cramming 11- year-olds for the national tests.

“By stressing the basics — reading and writing — and testing like crazy you reduce the level of cognitive stimulation. Children have the facts but they are not thinking very well,” says Adey. “And they are not getting hands-on physical experience of the way materials behave.”

Ginsburg says parents too can do their bit. “When did children stop playing with mud, plasticine and Meccano and start playing with Xboxes and computer games?” she asks. Parents should switch off the television and “sit children around the dinner table to debate issues such as ‘What should we have done about the whale in the Thames?’ ” says Adey.

If these experts are right — and our children are losing the ability to think, the burning question is: what is the value of what they are being taught in primary school and of all those test results that every year rise to new heights? Paul Black, professor of education at King’s College, London is one of the experts so startled by these findings that he now wants ministers to reassess what our children are being taught.

“The decline shown up by this research is big and it is worrying,” he says. “It casts doubt on claims that standards are improving . . . There is not much evidence, in fact I don’t know of any good evidence, that the things tested at the moment in national tests at the age of 11 and 14 are of long-term benefit to learning . . . The government should look at this again.”

The Qualifications and Curriculum Authority (QCA), the exams watchdog, has called in the research. Asked whether it may prompt changes in what is being taught in our schools, a spokesman said: “We are cautious about research where questions never change because times change and the world changes.”

And our children’s knowledge and intelligence is changing too — but not, perhaps, in the direction ministers would have us believe.