Yesterday, the Princess Royal, patron of the organisation which ran the tests, said part of the problem was that children were exposed to too much visual information at an early age.
"So much information is thrown at you visually when you are young, but television is the obvious one. They do not realise until later that they need other skills to transfer it," she said at the launch in London of the Adult Literacy and Basic Skills Unit's report, Older and Younger: the basic skills of different age groups.
The study by the unit was based on interviews with 3,000 people in England and Wales aged 22-24, 32-34, 42-44, 52-54, 62-64 and 72-74.
Tested on a range of everyday tasks, from understanding an advertisement for a concert to a fact sheet on hypothermia treatment, 15 per cent were found to have poor or very poor reading and numeracy skills.
Only one in five did all the reading tests correctly, and one in three completed the numeracy test correctly. Alan Wells, then unit's director, warned that this could be the tip of an iceberg. He said 2,874 interviewees attempted all the tasks, which was"likely to be an under-estimate of the number of people with poor basic skills because of the reluctance of some people to take part in the survey."
The worst performers were the 72- to 74-year-olds, but experts believe that ageing affects their ability.
Overall, the forties age group did best: just over a quarter had high literacy levels and more than a third high numeracy. Those in their thirties did marginally worse. Of the 22- to 24-year-olds, only 24 per cent achieved high literacy and 23 per cent high numeracy.
Mr Wells said this could be because they were less familiar with "life-skills and adult tasks", rather than a decline in standards.
Women fared worse than men. Although most believed they had fewer problems with reading, in tests 10 per cent were found to have lower standards compared with 7 per cent men, and around 23 per cent were poor on numeracy compared with 12 per cent men.
People from higher socio-economic groups did better than those from poorer backgrounds and the unemployed scored worse than the employed.
The Princess said: "There are probably some people who fall into the low-skills category who can work out the odds in a betting shop quicker than anyone else - it's a skill but it may not be applicable anywhere else." She said only by improving basic literacy and numeracy could those with low skills "open the doors of technology and information".
There was no quick fix to the long-term problems, Mr Wells said, but preventative programmes and family literacy projects helped. He added that the potential of "a significant number" of people would continue to be wasted "unless we can . . . reduce the number of people who have very serious difficulties with skills most of us take for granted".
David Blunkett, Labour's education spokesman, said yesterday that the Government was failing to do enough to improve literacy and numeracy.
"It is quite disgraceful that the Government is abandoning the reading-recovery scheme which tackles literacy problems at an early age when we quite clearly need to do more to improve these basic skills," he said.Reuse content