Theresa May should never have appointed someone as Britain’s top diplomat who had accused Barack Obama of being anti-British because he’s “part-Kenyan”, and notoriously wrote about “flag-waving piccaninnies”.

Now, after 16 months of the foreign secretary damaging Britain’s standing in the world, she should sack him.

With shocking callousness, Boris Johnson caused outrage last month by declaring that the Libyan city of Sirte could become “the next Dubai” once they “clear the dead bodies away”.

Making jokes about people killed in a civil war, (in which the Conservative-led government intervened militarily and which has made us less safe), is breathtakingly crass and was rightly condemned by Tory MPs as well as Libyan leaders.

His colonial throwback take on the world is completely out of step with the reality of our diverse and modern country and the views of our people. We should embrace the 21st century, not hanker after the 19th.

In his first conference speech as foreign secretary, Johnson referred to Africa, a continent of more than 50 states as “that country”. He claimed that life expectancy in Africa “has risen astonishingly” as it “has entered the global economic system”. Sadly, life expectancy has not risen astonishingly across the continent in the last 30 years and has fallen in several countries.

Britain’s top diplomat needs to be a leader in cultural sensitivity, but he repeatedly lets our country down.

He thought a Sikh gurdwara the appropriate place to discuss Indian whisky tariffs. Johnson was forced to apologise when it was pointed out that alcohol is prohibited in Sikhism.

On an official trip to Myanmar, Johnson was captured on film embarrassing his hosts by reciting a colonial era poem in a sacred Buddhist site. Britain’s ambassador Andrew Patrick was forced to intervene to tell him it was inappropriate. Once again, Johnson showed his nostalgia for a brutal imperial past.

Johnson has not learned how to be diplomatic or represent our country.

And now we have the heartbreaking case of Nazanin Zaghari-Ratcliffe, whose future liberty is under threat because of Johnson’s serial bungling. The foreign secretary should have the decency to say clearly and unequivocally that he was wrong and do everything possible to make sure she isn’t punished for his cavalier mistake.

With growing tensions on the Korean peninsula, we need serious diplomacy and nuclear armed states to re-engage with the process of meaningful multilateral disarmament. Johnson has failed to provide it, ducking our international obligations under the nuclear non-proliferation treaty.

We’ve put up with him embarrassing and undermining our country through his incompetence and putting our citizens at risk for long enough. It’s time for Boris Johnson to go.