The head of the US Food and Drug Administration has risked sparking a fresh conflict with Donald Trump over the timing of a coronavirus vaccine, after insisting his scientists would stick to a set of guidelines that the US president has threatened to overrule.

Stephen Hahn, FDA commissioner, said on Tuesday his officials would assess any vaccine application against a set of criteria that experts say make it all but impossible that one will be available before November’s election.

Those guidelines — which include a requirement that companies monitor half their trial participants for at least two months before making an application — have been the source of political contention since the president said last week he might not approve them. Mr Trump called them a “political move”.