Donald Trump reached for one of his favourite measures of manhood on Tuesday in the ongoing spat with his own Secretary of State, challenging Rex Tillerson to an IQ test.

Last week Mr Tillerson was forced to restate his loyalty to Mr Trump after leaks suggested he considering resigning over the summer and had in private described the President as a “moron”.

In typical style, Mr Trump said he would be happy to settle the question of his intelligence once and for all.

“I think it's fake news, but if he did that, I guess we'll have to compare IQ tests,” he told Forbes magazine. “And I can tell you who is going to win.”

Mr Trump’s words mark the latest humiliation for the man who serves as his envoy to the world. Mr Tillerson has become increasingly isolated while the President looks to the retired and serving generals who staff his national security apparatus for guidance on foreign policy.