Prime Minister Boris Johnson has blamed a "mutant algorithm" for this summer's exam results fiasco.

"I am afraid your grades were almost derailed by a mutant algorithm and I know how stressful that must have been," Mr Johnson told pupils at a school in Leicestershire.

In the fall-out from the exam problems, the Department for Education's permanent secretary has stepped down.

If follows the resignation of the head of Ofqual on Tuesday.

The National Education Union called Mr Johnson's comments "brazen" and accused the prime minister of trying to "idly shrug away a disaster that his own government created".

A statement on the departure of the permanent secretary, Jonathan Slater, said "the prime minister has concluded that there is a need for fresh official leadership" at the department.

The role as the department's most senior civil servant will be taken on in an interim basis by Susan Acland-Hood.