Secondary schools in Wales are performing worse since league tables were abolished, claims new research.
The naming and shaming that accompanied the publication of league tables led to more "effective" schools, says the Bristol University study.
League tables were abolished in 2001 in Wales but still exist in England.
The assembly government said league tables were not the "most effective" way of presenting schools' information. Unions have criticised the research.
Education in Wales is a devolved issue, run by the assembly government.
Researchers at Bristol University compared schools in England and Wales before and after the league table abolition in 2001.
Their research found that the lack of public naming and shaming of badly performing schools had removed the pressure upon them to improve.
This has led to schools in Wales being less effective than their English counterparts.
The result, according to the research, is that an average student in England will outperform an average student in Wales by two GCSE grades.
The Bristol University research document said: "We find that the reform significantly and systematically reduces school effectiveness."
"We find systematic, significant and robust evidence that abolishing school league tables markedly reduced school effectiveness in Wales.
"The impact is sizeable: a fall of 1.92 GCSE grades per student per year".
The university research also found that the abolition of league tables has had a marked effect on the worst performing schools.
"The lower 75% of schools are affected negatively with the poorest and lowest ability schools falling behind the most.
"Our results show that the policy reform in Wales reduced average performance and raised educational inequality."
The university research concludes that in light of the findings, the re-introduction of published league tables would be "an extremely cost-effective" way of raising educational achievement.
"Our results suggest that school accountability policies hold promise for raising school performance, particularly for students in disadvantaged schools and neighbourhoods.
"If uniform national test results exist, publishing these in a locally comparative format appears to be an extremely cost-effective policy for raising attainment and reducing inequalities in attainment."
An assembly government spokesperson defended the decision to abolish league tables said young people's levels of attainment in Wales continued to rise "year on year".
"We want to improve performance across all schools and believe strongly that league tables are not the most effective way of presenting information to schools, parents, and the wider public."
"Robust self evaluation and performance data play a vital role in promoting continuous improvement and we fully endorse this.
"We have a commitment to learning and benchmarking against those near and far away and the use of data at all levels in the system to promote continuous improvement and to monitor the effectiveness of our interventions.
'Slap in the face'
Education Minister Leighton Andrews added: "In Wales over the decade of devolution we have implemented most of the changes the profession wanted to see. So we don't have league tables.
"We will see in December when the international comparisons of school performance are reported whether that approach has paid off."
The NAS/UWT teaching union claimed the research was "flawed" and no definitive conclusions could be drawn.
"This appears to be an ideological theory in desperate search of evidence to back it up," Chris Keates, said general secretary.
"It conveniently fails to highlight the fact that overall school performance in schools in Wales increased during the period covered by the report."
David Reynolds, professor of education at Plymouth University and former adviser to the Welsh Assembly Government and the Department of Education, said it would be "hard to fault" the study.
"This data is based on individual pupils and they have been able to follow a cohort of children through in both England and Wales," said Prof Reynolds.
"It seems the schools at the bottom of the achievement hierarchy in Wales are performing poorly because they just don't face the pressure faced by schools in England".
"This is a slap in the face with good data. The effect appears to be a couple of grades per pupil at GCSE level. I think they (the assembly government) should re-evaluate and think again.
"League tables were abolished for the best possible reasons but now they should have another look."