By Ros Urwin

Boris Johnson is a dilettante, trifling with diplomacy. If that were ever in doubt – or if anyone again attempts the 'intellectual passing himself off as Bertie Wooster' defence – we need only consider his handling of the case of Nazanin Zaghari-Ratcliffe, an Anglo-Iranian dual national imprisoned in Tehran.

The foreign secretary, who should be fighting for her release, has instead risked prolonging her sentence, showing the same level of care when it comes to British citizens detained abroad as he did when he shoulder-barged a ten-year-old in a game of street rugby.

When Theresa May made Johnson foreign secretary, I briefly wondered if she meant it as a punishment instead of a promotion – a Danaian gift. It cost him around £750,000 in the first year – he had to back out of a £500,000 deal with Hodder & Stoughton to write a biography of Shakespeare and drop his lucrative Telegraph column. But it has proved one of May's worst decisions, against considerable competition – like promoting someone who calls themselves a 'Bantersaurus Rex' to one of the great offices of state.

Johnson is an embarrassment internationally. In everything he does, he gives off the air of the essay crisis, the private school pupil who hasn't done his homework and is winging it. He is not a man of detail. His unchecked tongue – amusing to many a backbencher – is now our insulting emissary to the world. Prince Philip has more tact. There was nothing funny about Johnson's claim that the Libyan city of Sirte could be the new Dubai if they "cleared the dead bodies away".

But this situation is even worse. Johnson stated incorrectly before a parliamentary committee that Zaghari-Ratcliffe had been "teaching people journalism" when she was arrested in Iran last year. She and her daughter Gabriella were actually returning from a holiday visiting her parents – an integral part of her defence – when she was detained by the Revolutionary Guard at Tehran airport. Although Zaghari-Ratcliffe works for the Thomson Reuters Foundation, she is neither a journalist nor a journalism teacher. Which Johnson should have known.

This was no 'Bo-Jo gaffe', as Liam Fox has tried to shrug it off as today. The job of a foreign secretary is to defend British citizens abroad and yet Johnson has left one at risk of another five years in an Iranian jail. He should know that careless talk costs lives.

As Tulip Siddiq, Zaghari-Ratcliffe's local MP, told me this morning:

"I've raised this case repeatedly in parliament – in written questions, in Westminster Hall debates, in letters to Boris Johnson and the prime minister. I've made it absolutely clear every time that Nazanin and Gabriella were on holiday in Iran. I'm gobsmacked that Boris Johnson did not bother finding out the details of this very important case which has been ongoing for 18 months now. I know he is let off lightly for his gaffes because people find him funny, but this isn't funny. This is a critical mistake."

After Johnson's comments, Zaghari-Ratcliffe was summoned to court on Saturday. A day later, an official Iranian website said that Johnson had thrown "new light" on her case. The Iranian regime is using his words – the words of a British foreign secretary – against a British citizen. Johnson didn't even have the decency to act quickly to fix his mistake. Instead, a mealy-mouthed statement came from the Foreign Office, saying he was trying to help.

Zaghari-Ratcliffe's husband, Richard, now wants Johnson to make a statement to parliament retracting his incorrect comments. Ratcliffe has already been underserved by the Foreign Office, whose representatives have repeatedly told him they don't want to give the case a higher profile as he lobbies to raise awareness of it. He has described what is happening to his wife to me as "torture" and notes that business deals are being signed with Iran even as she languishes in prison. Before Johnson's statement, Ratcliffe was repeatedly passed around a bureaucratic circle. The case would go to the more junior Foreign Office minister Alistair Burt, who would say he would raise it with Johnson, whose office would then hand it back to Burt.

Johnson's incompetence has now left his own political future in the hands of the Iranians. But if he survives this, we must stop indulging him. Let's stop downplaying his errors as 'gaffes'. Let's stop 'Boris is Boris' being an excuse for failings that would get other ministers sacked. Let's stop calling him 'Boris', which reflects a degree of affection that is wholly underserved. He's not Madonna. He's not even bloody Chico. But he has – like Donald Trump in the US – used his celebrity as a form of privilege to keep him in post.

Every time Johnson has been tested, he has been found wanting. As London mayor, there were the vanity projects – the 'dangleway to nowhere' cablecar, the thankfully never-built Boris airport island – but when the capital needed its mayor during the 2011 riots, he took three days to return from his holiday. And yet he was welcomed back to parliament and back into the cabinet.

Johnson is a man whose every failure goes rewarded. But how much longer will we have to put up with a foreign secretary who keeps hurting Britain and Britons abroad?

Rosamund Urwin is a columnist for the London Evening Standard and is writing – painfully slowly – her first book.

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