Students from the 300 schools were the first affected by the medium of instruction policy introduced by the government in 1998. This policy forced all but 112 secondary schools to adopt Chinese as their medium of instruction, with the belief that most students would learn more effectively in their mother tongue.

The analysis also found that the overall performance of English-medium schools had either dropped since last year or remained the same.

The Hong Kong Examinations and Assessment Authority declined to make comparisons between the performance of the two types of schools, explaining that the academic standards of pupils in Chinese-medium schools were generally much lower.

It also refused to disclose the exact percentage increase or drop in the pass rates, prompting some educators to doubt whether the differences were significant.

Fanny Lam Fan Kit-fong, principal education officer at the Education and Manpower Bureau, said the decline in students' English standards could not be considered to be a failure of the language policy.

'There are many factors affecting students' learning of English, including motivation, teaching strategies and parental support. We cannot just blame it on the use of mother-tongue education,' she said, adding that the government would identify the factors causing a rise in the overall pass rate in English in about 40 Chinese-medium schools.

Ms Lam said the analysis carried positive implications for the policy. 'Our conclusion is that the policy has helped raise students' learning effectiveness and performance,' she said.

'The findings show that learning in English is not necessarily good for some students. I believe only 20 to 30 per cent of schools have the ability to teach in English.'

Analysis of this year's HKCEE results will form part of the government's one-year review of the language policy.

But the findings prompted criticism from Chinese-medium schools about the lack of government support to help students raise their English proficiency.

Choi Kwok-kwong, vice-principal of Yan Oi Tong Tin Ka Ping Secondary School, which was forced to switch from English to Chinese five years ago, said it was the government's responsibility to provide additional support to raise the English standards of students at Chinese-medium schools.

'Students receiving mother-tongue education only need to use English during English lessons. We cannot expect them to have a high proficiency of the language when the government has done nothing to adjust the teaching methods and curriculum for them,' he said.

Rosalind Chan Lo-sai, chairwoman of the Association of English-Medium Secondary Schools, said she believed the drop in pass rates among English-medium schools was only marginal.

She argued that it was natural for English-medium schools to lag slightly behind in terms of subject knowledge, as they had to spend more time learning English.