Students who use computers a lot at school have worse maths and reading performance, research suggests.
Millions of pounds is spent equipping schools
Those using computers several times a week performed "sizeably and statistically significantly worse" than those who used them less often.
A simpler interpretation of a major international study of 15 year olds had suggested more computers meant better performance, the researchers say.
Last week Prince Charles complained of "computer-driven modules" in education.
In a letter to last week's Association of Colleges conference, he said: "I simply do not believe that passion for subject or skill, combined with inspiring teaching, can be replaced by computer-driven modules, which seem to occupy a disproportionate amount of current practice."
The UK government's computer agency, Becta, stresses that computers are a tool, not something to supplant other teaching.
The new study was done by Thomas Fuchs and Ludger Woessmann of the CESifo economic research organisation in Munich.
They used the test performance and background data from the 2000 PISA study involving tens of thousands of students in 31 countries, including the UK, organised by the Organisation for Economic Co-operation and Development (OECD).
When it was published, the OECD warned about the possible limitations of its findings but pointed to a positive relationship between students' interest in computers and their literacy.
The belief that there is an educational benefit - and not just better work skills - has underpinned huge investment by governments, and many parents, in information and communication technology (ICT).
Fuchs and Woessmann found that the more computers there were in students' homes, the better their test performance.
But more computers went with more affluent, better-educated families. So they took this into account in the statistical analysis.
The result: the more computers in a student's home, the worse the student's maths performance.
In schools, they found students performed worse in those which reported a significant lack of computers.
But again, once they took into account the schools' general resources the same pattern emerged.
"That is, the initial positive pattern on computer availability at school simply reflects that schools with better computer availability also feature other positive school characteristics."
Once these were taken into account, computer availability was not related to student performance.
They then considered computer use, particularly internet access, e-mail and educational software.
At home, greater use went with better test performance. And those who used these the least did significantly worse.
But in schools the effect was different.
Students who hardly ever used computers did a little worse than those who used them between a few times a year and several times a month.
But those who used computers at school several times a week performed "sizeably and statistically significantly worse" in both maths and reading.
The researchers say their analysis just describes what the statistics show without explaining the findings. But they suggest two theories.
One is "ability bias" - it might be that teachers do not want low-ability students to use computers.
But this is less likely to account for the impact of high usage - which might instead be "a true negative effect of excessive computer use".
And it might be that some computerised learning is beneficial but at higher intensities it crowds out more effective teaching methods and hinders students' creativity.
The UK government's computer agency, Becta, takes issue with the study's findings.
Dr Tim Rudd, from the organisation told the BBC News website: "There is evidence to suggest that ICT can be a very powerful tool for developing literacy skills but that this may vary across ages and in relation to different aspects of literacy.
"In maths however, the evidence appears stronger. In a recent Becta paper analysing available research about primary and secondary teachers' use of ICT in maths, key findings suggest that ICT has changed the nature of teaching and learning.
"A wide range of tools is now available that enables learning to take place in a way that is more dynamic and powerful."
Computers and Student Learning: bivariate and multivariate evidence on the availability and use of computers at home and at school, by Thomas Fuchs and Ludger Woessmann, CESifo working paper no. 1321.