Press release from DfE and discussion of underlying statistics in fullfact.org. A table from fullfact.org
based on Trends in International Mathematics and Science Study (TIMSS):
From TIMSS 2007: England rank bottom of all sampled countries for restricting calculator use in the maths classroom
De Morgan Gazette 
Judging by comments on CBBC, banning calculators in tests could be unpopular with children.
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