Labour Party leader Jeremy Corbyn has called on Prime Minister Theresa May to sack foreign secretary Boris Johnson following his 'deeply regrettable error' relating to Briton Nazanin Zaghari-Ratcliffe's detention in Iran.

Johnson falsely stated that Zaghari-Ratcliffe, who is serving a five-year sentence for spying, had been teaching people journalism in Iran. Her family has always insisted she was on holiday in the country.

Iranian state television interpreted Johnson's comments as a "confession".

In a column for The Guardian, Corbyn said Zaghari-Ratcliffe's future was "under threat because of Johnson's serial bungling".

Johnson was forced to apologise for his comment, saying he "could have been clearer".

The Labour leader's comments on the incident followed a list of previous incidents involving Johnson in which he accused Barack Obama of being anti-British because he's "part-Kenyan" and joked about clearing away dead bodies in the Libyan city of Sirte.

Corbyn also references Johnson referring to black people as "piccaninnies" and talking about "watermelon smiles" in a 2002 column for the Daily Telegraph, and referring to Africa as "that country".

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

Corbyn also said: "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."

In a separate column for The Times, Corbyn attacked May's government over Brexit ahead of the return of the EU Withdrawal Bill - which he calls "an undemocratic government power-grab".

"The Brexit talks are at a crucial point. The country cannot afford the crippling splits and indecision at the heart of government," he says. "The government can't give a lead because the cabinet is split down the middle, spending more time negotiating with each other than the EU."

Speaking of May, whose position has been significantly weakened since the General Election she called backfired and her government lost its majority, Corbyn says: "If she can't get a grip, she should move out of the way and let Labour deliver a Brexit that works for the many, not the few.

"Labour wants to see a new cooperative relationship with Europe. We have proposed a time-limited transitional deal on the same basic terms as now. Such arrangements will not frustrate Brexit but will make possible a Brexit that puts jobs and living standards first."