Classes are only truly effective if they are taken during childhood, claims Professor Anna Vignoles, of the Institute of Education, part of the University of London.
She says many low level qualifications fail to boost "productivity and earnings".
The Government has pumped millions of pounds into improving adults' reading, writing and maths to help them get and keep jobs.
Ministers insist it is vital that adults have skills to find work during the recession, helping to boost Britain's economic competitiveness.
But speaking at an Institute for Fiscal Studies conference in Cambridge, she will tell how the UK is still in the bottom half of international basic skills tables produced by the respected Organisation for Economic Co-operation and Development.
She says: "It is well known that an individual's basic skills level affects how much they earn, but research shows that the three Rs are best acquired in childhood.
"Policies and qualifications to help adults develop them have proved largely ineffective."
Prof Vignoles says figures from the National Audit Office show £995 million was spent on the Government's Skills For Life programme - aimed at improving basic skills - in 2006/07. A course costs an average of £500, she says.
Prof Vignoles says short training programmes of up to 20 hours may be worthwhile because they reach people without any other formal education.
But she adds: "The array of low-level qualifications available to adults has not boosted productivity and earnings. Gains from workplace courses are particularly small."
Adults need intensive programmes lasting between 100 and 150 hours to build their skills, she says, which could be costly.
"Adult basic skills training might increase equality of opportunity, but unfortunately it won't boost economic competitiveness," Prof Vignoles says.