House of Commons speaker John Bercow's extraordinary decision to "strongly oppose" Donald Trump addressing the British Parliament during a state visit next month reveals a critical flaw in the tactics adopted by many critics of the US President.

In an unprecedented intervention by a Speaker of the Commons, Bercow, who was a moderate Conservative before taking up the politically neutral role, gave a short speech decrying Trump's "migrant ban" in response to a point of order made by a Labour MP.