Hugely popular brain training games that sell in their millions may be fun to play but do nothing to improve IQ, a study has found.
The hand-held computer games are said to "exercise" the mental muscles and over time improve general thinking and memory.
But a study involving more than 11,400 participants across the UK concluded there was "no evidence" that they can.
Although practice improved game performance, the skills learned did not transfer to unrehearsed mental tasks.
Players did no better at a series of independent mental tests than another group of volunteers given general knowledge quizzes instead of brain training.
This was true even when the tests tapped into the same areas of the brain used to play the games.
Dr Adrian Owen, one of the researchers from the Medical Research Council Cognition and Brain Sciences Unit in Cambridge, said: "Brain training, or the quest to improve brain function through regular use of computer tests, is a multimillion-pound industry, yet up until now there's been a real lack of robust evidence to show it really works.
"Our findings will no doubt surprise millions of people worldwide who do some form of brain training every day in the belief that 'exercising' their brain makes them better at everyday thinking tasks.
"In one of our computer games that tests memory by assessing how many numbers could be remembered by players, we found it would take almost four years of playing brain training games regularly each week to remember just one extra digit."
The study was carried out in collaboration with the BBC science programme "Bang Goes The Theory".
Viewers aged 18 to 60 took part in a six-week study of brain training via the BBC mass participation science website Lab UK.
A total of 11,430 volunteers trained weekly on typical game tasks aimed at improving reasoning, memory, planning, visual-spatial skills and attention.
At the beginning and end of the six-week training period, participants were given four benchmarking tests designed by scientists to be sensitive to changes in brain function.
Volunteers were split randomly into three groups, two of which practised six training games for at least 10 minutes a day three times a week. The third "control" group, used for comparison, answered general knowledge questions on the website but did not undergo any brain training.
The first group's training focussed on reasoning, planning and problem solving, while the second engaged a broad set of mental abilities commonly seen in commercial brain training games.
As participants improved their skills, the difficulty of the tasks was increased. On average, they completed 24.47 training sessions each.
Both sets of brain training volunteers showed regular improvements in the benchmarking tests, but performed no better than the control group.
In two tests, the quiz participants actually improved more than the game players.
The findings are published online by the journal Nature today. They will also be featured in a one-hour programme "Can You Train Your Brain? A Bang Goes the Theory Special" to be broadcast on BBC One at 9pm tonight.
It has been suggested that brain training games might help prevent age-related mental decline.
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "This evidence could change the way we look at brain training games and shows staying active by taking a walk, for example, is a better use of our time.
"The next question is whether brain training can help maintain your brain as you get older."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "This suggests that 'brain training' does not improve people's cognitive ability. More research will tell us if these games have any effect on cognition as we age."
Dallas Campbell, presenter of Bang Goes the Theory, said: "Millions of us play brain training games on the assumption that expensive games make us better at everyday thinking tasks, but until now there's been no evidence to prove that these games work at all.. We set out to gather real, scientific evidence that would answer the question of whether these games are worth our money. And now we have our answer."
Some previous studies of older people and pre-school children had shown modest positive effects from brain training, said the scientists.
People who regularly played video games also out-performed non-players in some visual attention tests. However, there was no scientific evidence to support the widely held belief that brain-training computer games "improve general cognitive function in the wider population".