A report 'Early entry to GCSE examinations' has been published by the Department for Education (DfE).
The DfE says:
There has been a significant increase in the number of pupils being entered early for GCSEs particularly in Mathematics and English. Whilst there has been a long history of this practice for the highest achieving pupils, the trend is increasing for pupils of all abilities. For many, this can be detrimental to their overall performance.
The Advisory Committee on Mathematics Education (ACME) released it's own report 'Early and Multiple Entry to GCSE Mathematics' on this matter in May 2011.