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Plans to get all children back to school in early September are now at “serious risk” because of government incompetence and the chaos caused by the exams fiasco, the Labour leader Keir Starmer has warned.

In one of his strongest interventions to date, which is bound to draw a furious response from Downing Street, Starmer told the Observer that two crucial weeks, which should have been spent preparing for schools to reopen, have been wasted dealing with a self-inflicted “mess” that has destroyed public confidence in government.