More than 45,000 pupils - around one in 14 students in England, Wales and Northern Ireland - will resit their English GCSE exams next month as a result of the row over this summer's grade boundaries.
Headteachers said it was a "gross injustice" that students will have to do resits after a scandal that was not their fault.
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The Labour leader said newspapers were going "a little bit James Bond" over the "increasingly wild and entirely false" claims.
The "astonishing" find was made during an excavation at Vindolanda, near Hexham in Northumberland.