Some schools are "playing the system" by unfairly entering Welsh-speaking candidates for the easier GCSEs designed for second-language pupils. A team of academics say schools which enter strong Welsh speakers for the simpler paper are placing other candidates at a disadvantage.
Consultants Llais y Lli were asked to investigate moves away from first to second-language Welsh in schools between key stages 2 and 3 by ACCAC, the Welsh qualifications, curriculum and assessment authority for Wales.
They found a "substantial" 22 per cent shift from first to second-language Welsh over a four-year period - particularly in traditional Welsh strongholds.
Welsh-speaking parents - who lacked confidence in their children's ability to gain high grades on the first-language paper - were also encouraging the trend, according to their report. Interviews with parents during the research revealed that bullying and peer pressure were also major factors in deciding whether their child moved to second-language Welsh.
The report recommends the Assembly gives advice to schools and heads on sensible and positive guidance to pupils.
Jane Davidson, education and lifelong learning minister, this week agreed more needed to be done to ensure continuity in Welsh learning.
She said it was unfair of schools to enter pupils for the second-language paper just to gain better results.
Speaking at a meeting of the Assembly's education and lifelong learning committee, she said a new definition of Welsh-medium education, due to be published later this month, would help schools clarify their Welsh provision.
And an Assembly government spokesman said it also planned to produce guidelines on continuity.
But Gruff Hughes, general secretary of Welsh-medium teachers' union UCAC, said: "There is still a belief that Welsh first-language pupils need a good command of English to get on."
He called for Welsh second-language papers to be written in Welsh, and for marks to take account of pupils' home language.
The research team studied 72 secondary and 153 primary schools. They found in bilingual schools where there had been a big shift towards second-language Welsh, GCSE results were 20 per cent higher than the Wales average.
Out of 24,988 pupils assessed in first-language Welsh at KS2, 19,405 went on to take first-language exams at KS3 - with more than a fifth (5,583) sitting second-language papers.
The greatest movement took place in the staunch Welsh-speaking counties of Carmarthenshire, Ceredigion, and Conwy.
In Conwy, the county with the greatest movement, just 48.5 per cent of those assessed in Welsh at KS2 stayed first language at KS3.
ACCAC told TES Cymru some of the points raised in the report were the opinions of the authors.
The Movement of Pupils Between Welsh and Second Language Welsh by Helini Gruffudd, Dr Elin Meek and Catrin Stevens, see www.wales.gov.uk